import types
import sys
from itertools import izip

import django.db.models.manager     # Imported to register signal handler.
from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned, FieldError, ValidationError, NON_FIELD_ERRORS
from django.core import validators
from django.db.models.fields import AutoField, FieldDoesNotExist
from django.db.models.fields.related import (OneToOneRel, ManyToOneRel,
    OneToOneField, add_lazy_relation)
from django.db.models.query import delete_objects, Q
from django.db.models.query_utils import CollectedObjects, DeferredAttribute
from django.db.models.options import Options
from django.db import connections, router, transaction, DatabaseError, DEFAULT_DB_ALIAS
from django.db.models import signals
from django.db.models.loading import register_models, get_model
from django.utils.translation import ugettext_lazy as _
import django.utils.copycompat as copy
from django.utils.functional import curry, update_wrapper
from django.utils.encoding import smart_str, force_unicode
from django.utils.text import get_text_list, capfirst
from django.conf import settings

class ModelBase(type):
    """
    Metaclass for all models.
    """
    def __new__(cls, name, bases, attrs):
        super_new = super(ModelBase, cls).__new__
        parents = [b for b in bases if isinstance(b, ModelBase)]
        if not parents:
            # If this isn't a subclass of Model, don't do anything special.
            return super_new(cls, name, bases, attrs)

        # Create the class.
        module = attrs.pop('__module__')
        new_class = super_new(cls, name, bases, {'__module__': module})
        attr_meta = attrs.pop('Meta', None)
        abstract = getattr(attr_meta, 'abstract', False)
        if not attr_meta:
            meta = getattr(new_class, 'Meta', None)
        else:
            meta = attr_meta
        base_meta = getattr(new_class, '_meta', None)

        if getattr(meta, 'app_label', None) is None:
            # Figure out the app_label by looking one level up.
            # For 'django.contrib.sites.models', this would be 'sites'.
            model_module = sys.modules[new_class.__module__]
            kwargs = {"app_label": model_module.__name__.split('.')[-2]}
        else:
            kwargs = {}

        new_class.add_to_class('_meta', Options(meta, **kwargs))
        if not abstract:
            new_class.add_to_class('DoesNotExist', subclass_exception('DoesNotExist',
                    tuple(x.DoesNotExist
                            for x in parents if hasattr(x, '_meta') and not x._meta.abstract)
                                    or (ObjectDoesNotExist,), module))
            new_class.add_to_class('MultipleObjectsReturned', subclass_exception('MultipleObjectsReturned',
                    tuple(x.MultipleObjectsReturned
                            for x in parents if hasattr(x, '_meta') and not x._meta.abstract)
                                    or (MultipleObjectsReturned,), module))
            if base_meta and not base_meta.abstract:
                # Non-abstract child classes inherit some attributes from their
                # non-abstract parent (unless an ABC comes before it in the
                # method resolution order).
                if not hasattr(meta, 'ordering'):
                    new_class._meta.ordering = base_meta.ordering
                if not hasattr(meta, 'get_latest_by'):
                    new_class._meta.get_latest_by = base_meta.get_latest_by

        is_proxy = new_class._meta.proxy

        if getattr(new_class, '_default_manager', None):
            if not is_proxy:
                # Multi-table inheritance doesn't inherit default manager from
                # parents.
                new_class._default_manager = None
                new_class._base_manager = None
            else:
                # Proxy classes do inherit parent's default manager, if none is
                # set explicitly.
                new_class._default_manager = new_class._default_manager._copy_to_model(new_class)
                new_class._base_manager = new_class._base_manager._copy_to_model(new_class)

        # Bail out early if we have already created this class.
        m = get_model(new_class._meta.app_label, name, False)
        if m is not None:
            return m

        # Add all attributes to the class.
        for obj_name, obj in attrs.items():
            new_class.add_to_class(obj_name, obj)

        # All the fields of any type declared on this model
        new_fields = new_class._meta.local_fields + \
                     new_class._meta.local_many_to_many + \
                     new_class._meta.virtual_fields
        field_names = set([f.name for f in new_fields])

        # Basic setup for proxy models.
        if is_proxy:
            base = None
            for parent in [cls for cls in parents if hasattr(cls, '_meta')]:
                if parent._meta.abstract:
                    if parent._meta.fields:
                        raise TypeError("Abstract base class containing model fields not permitted for proxy model '%s'." % name)
                    else:
                        continue
                if base is not None:
                    raise TypeError("Proxy model '%s' has more than one non-abstract model base class." % name)
                else:
                    base = parent
            if base is None:
                    raise TypeError("Proxy model '%s' has no non-abstract model base class." % name)
            if (new_class._meta.local_fields or
                    new_class._meta.local_many_to_many):
                raise FieldError("Proxy model '%s' contains model fields." % name)
            while base._meta.proxy:
                base = base._meta.proxy_for_model
            new_class._meta.setup_proxy(base)

        # Do the appropriate setup for any model parents.
        o2o_map = dict([(f.rel.to, f) for f in new_class._meta.local_fields
                if isinstance(f, OneToOneField)])

        for base in parents:
            original_base = base
            if not hasattr(base, '_meta'):
                # Things without _meta aren't functional models, so they're
                # uninteresting parents.
                continue

            parent_fields = base._meta.local_fields + base._meta.local_many_to_many
            # Check for clashes between locally declared fields and those
            # on the base classes (we cannot handle shadowed fields at the
            # moment).
            for field in parent_fields:
                if field.name in field_names:
                    raise FieldError('Local field %r in class %r clashes '
                                     'with field of similar name from '
                                     'base class %r' %
                                        (field.name, name, base.__name__))
            if not base._meta.abstract:
                # Concrete classes...
                while base._meta.proxy:
                    # Skip over a proxy class to the "real" base it proxies.
                    base = base._meta.proxy_for_model
                if base in o2o_map:
                    field = o2o_map[base]
                elif not is_proxy:
                    attr_name = '%s_ptr' % base._meta.module_name
                    field = OneToOneField(base, name=attr_name,
                            auto_created=True, parent_link=True)
                    new_class.add_to_class(attr_name, field)
                else:
                    field = None
                new_class._meta.parents[base] = field
            else:
                # .. and abstract ones.
                for field in parent_fields:
                    new_class.add_to_class(field.name, copy.deepcopy(field))

                # Pass any non-abstract parent classes onto child.
                new_class._meta.parents.update(base._meta.parents)

            # Inherit managers from the abstract base classes.
            new_class.copy_managers(base._meta.abstract_managers)

            # Proxy models inherit the non-abstract managers from their base,
            # unless they have redefined any of them.
            if is_proxy:
                new_class.copy_managers(original_base._meta.concrete_managers)

            # Inherit virtual fields (like GenericForeignKey) from the parent
            # class
            for field in base._meta.virtual_fields:
                if base._meta.abstract and field.name in field_names:
                    raise FieldError('Local field %r in class %r clashes '\
                                     'with field of similar name from '\
                                     'abstract base class %r' % \
                                        (field.name, name, base.__name__))
                new_class.add_to_class(field.name, copy.deepcopy(field))

        if abstract:
            # Abstract base models can't be instantiated and don't appear in
            # the list of models for an app. We do the final setup for them a
            # little differently from normal models.
            attr_meta.abstract = False
            new_class.Meta = attr_meta
            return new_class

        new_class._prepare()
        register_models(new_class._meta.app_label, new_class)

        # Because of the way imports happen (recursively), we may or may not be
        # the first time this model tries to register with the framework. There
        # should only be one class for each model, so we always return the
        # registered version.
        return get_model(new_class._meta.app_label, name, False)

    def copy_managers(cls, base_managers):
        # This is in-place sorting of an Options attribute, but that's fine.
        base_managers.sort()
        for _, mgr_name, manager in base_managers:
            val = getattr(cls, mgr_name, None)
            if not val or val is manager:
                new_manager = manager._copy_to_model(cls)
                cls.add_to_class(mgr_name, new_manager)

    def add_to_class(cls, name, value):
        if hasattr(value, 'contribute_to_class'):
            value.contribute_to_class(cls, name)
        else:
            setattr(cls, name, value)

    def _prepare(cls):
        """
        Creates some methods once self._meta has been populated.
        """
        opts = cls._meta
        opts._prepare(cls)

        if opts.order_with_respect_to:
            cls.get_next_in_order = curry(cls._get_next_or_previous_in_order, is_next=True)
            cls.get_previous_in_order = curry(cls._get_next_or_previous_in_order, is_next=False)
            # defer creating accessors on the foreign class until we are
            # certain it has been created
            def make_foreign_order_accessors(field, model, cls):
                setattr(
                    field.rel.to,
                    'get_%s_order' % cls.__name__.lower(),
                    curry(method_get_order, cls)
                )
                setattr(
                    field.rel.to,
                    'set_%s_order' % cls.__name__.lower(),
                    curry(method_set_order, cls)
                )
            add_lazy_relation(
                cls,
                opts.order_with_respect_to,
                opts.order_with_respect_to.rel.to,
                make_foreign_order_accessors
            )

        # Give the class a docstring -- its definition.
        if cls.__doc__ is None:
            cls.__doc__ = "%s(%s)" % (cls.__name__, ", ".join([f.attname for f in opts.fields]))

        if hasattr(cls, 'get_absolute_url'):
            cls.get_absolute_url = update_wrapper(curry(get_absolute_url, opts, cls.get_absolute_url),
                                                  cls.get_absolute_url)

        signals.class_prepared.send(sender=cls)

class ModelState(object):
    """
    A class for storing instance state
    """
    def __init__(self, db=None):
        self.db = db
        # If true, uniqueness validation checks will consider this a new, as-yet-unsaved object.
        # Necessary for correct validation of new instances of objects with explicit (non-auto) PKs.
        # This impacts validation only; it has no effect on the actual save.
        self.adding = True

class Model(object):
    __metaclass__ = ModelBase
    _deferred = False

    def __init__(self, *args, **kwargs):
        signals.pre_init.send(sender=self.__class__, args=args, kwargs=kwargs)

        # Set up the storage for instance state
        self._state = ModelState()

        # There is a rather weird disparity here; if kwargs, it's set, then args
        # overrides it. It should be one or the other; don't duplicate the work
        # The reason for the kwargs check is that standard iterator passes in by
        # args, and instantiation for iteration is 33% faster.
        args_len = len(args)
        if args_len > len(self._meta.fields):
            # Daft, but matches old exception sans the err msg.
            raise IndexError("Number of args exceeds number of fields")

        fields_iter = iter(self._meta.fields)
        if not kwargs:
            # The ordering of the izip calls matter - izip throws StopIteration
            # when an iter throws it. So if the first iter throws it, the second
            # is *not* consumed. We rely on this, so don't change the order
            # without changing the logic.
            for val, field in izip(args, fields_iter):
                setattr(self, field.attname, val)
        else:
            # Slower, kwargs-ready version.
            for val, field in izip(args, fields_iter):
                setattr(self, field.attname, val)
                kwargs.pop(field.name, None)
                # Maintain compatibility with existing calls.
                if isinstance(field.rel, ManyToOneRel):
                    kwargs.pop(field.attname, None)

        # Now we're left with the unprocessed fields that *must* come from
        # keywords, or default.

        for field in fields_iter:
            is_related_object = False
            # This slightly odd construct is so that we can access any
            # data-descriptor object (DeferredAttribute) without triggering its
            # __get__ method.
            if (field.attname not in kwargs and
                    isinstance(self.__class__.__dict__.get(field.attname), DeferredAttribute)):
                # This field will be populated on request.
                continue
            if kwargs:
                if isinstance(field.rel, ManyToOneRel):
                    try:
                        # Assume object instance was passed in.
                        rel_obj = kwargs.pop(field.name)
                        is_related_object = True
                    except KeyError:
                        try:
                            # Object instance wasn't passed in -- must be an ID.
                            val = kwargs.pop(field.attname)
                        except KeyError:
                            val = field.get_default()
                    else:
                        # Object instance was passed in. Special case: You can
                        # pass in "None" for related objects if it's allowed.
                        if rel_obj is None and field.null:
                            val = None
                else:
                    try:
                        val = kwargs.pop(field.attname)
                    except KeyError:
                        # This is done with an exception rather than the
                        # default argument on pop because we don't want
                        # get_default() to be evaluated, and then not used.
                        # Refs #12057.
                        val = field.get_default()
            else:
                val = field.get_default()
            if is_related_object:
                # If we are passed a related instance, set it using the
                # field.name instead of field.attname (e.g. "user" instead of
                # "user_id") so that the object gets properly cached (and type
                # checked) by the RelatedObjectDescriptor.
                setattr(self, field.name, rel_obj)
            else:
                setattr(self, field.attname, val)

        if kwargs:
            for prop in kwargs.keys():
                try:
                    if isinstance(getattr(self.__class__, prop), property):
                        setattr(self, prop, kwargs.pop(prop))
                except AttributeError:
                    pass
            if kwargs:
                raise TypeError("'%s' is an invalid keyword argument for this function" % kwargs.keys()[0])
        super(Model, self).__init__()
        signals.post_init.send(sender=self.__class__, instance=self)

    def __repr__(self):
        try:
            u = unicode(self)
        except (UnicodeEncodeError, UnicodeDecodeError):
            u = '[Bad Unicode data]'
        return smart_str(u'<%s: %s>' % (self.__class__.__name__, u))

    def __str__(self):
        if hasattr(self, '__unicode__'):
            return force_unicode(self).encode('utf-8')
        return '%s object' % self.__class__.__name__

    def __eq__(self, other):
        return isinstance(other, self.__class__) and self._get_pk_val() == other._get_pk_val()

    def __ne__(self, other):
        return not self.__eq__(other)

    def __hash__(self):
        return hash(self._get_pk_val())

    def __reduce__(self):
        """
        Provide pickling support. Normally, this just dispatches to Python's
        standard handling. However, for models with deferred field loading, we
        need to do things manually, as they're dynamically created classes and
        only module-level classes can be pickled by the default path.
        """
        data = self.__dict__
        model = self.__class__
        # The obvious thing to do here is to invoke super().__reduce__()
        # for the non-deferred case. Don't do that.
        # On Python 2.4, there is something wierd with __reduce__,
        # and as a result, the super call will cause an infinite recursion.
        # See #10547 and #12121.
        defers = []
        pk_val = None
        if self._deferred:
            from django.db.models.query_utils import deferred_class_factory
            factory = deferred_class_factory
            for field in self._meta.fields:
                if isinstance(self.__class__.__dict__.get(field.attname),
                        DeferredAttribute):
                    defers.append(field.attname)
                    if pk_val is None:
                        # The pk_val and model values are the same for all
                        # DeferredAttribute classes, so we only need to do this
                        # once.
                        obj = self.__class__.__dict__[field.attname]
                        model = obj.model_ref()
        else:
            factory = simple_class_factory
        return (model_unpickle, (model, defers, factory), data)

    def _get_pk_val(self, meta=None):
        if not meta:
            meta = self._meta
        return getattr(self, meta.pk.attname)

    def _set_pk_val(self, value):
        return setattr(self, self._meta.pk.attname, value)

    pk = property(_get_pk_val, _set_pk_val)

    def serializable_value(self, field_name):
        """
        Returns the value of the field name for this instance. If the field is
        a foreign key, returns the id value, instead of the object. If there's
        no Field object with this name on the model, the model attribute's
        value is returned directly.

        Used to serialize a field's value (in the serializer, or form output,
        for example). Normally, you would just access the attribute directly
        and not use this method.
        """
        try:
            field = self._meta.get_field_by_name(field_name)[0]
        except FieldDoesNotExist:
            return getattr(self, field_name)
        return getattr(self, field.attname)

    def save(self, force_insert=False, force_update=False, using=None):
        """
        Saves the current instance. Override this in a subclass if you want to
        control the saving process.

        The 'force_insert' and 'force_update' parameters can be used to insist
        that the "save" must be an SQL insert or update (or equivalent for
        non-SQL backends), respectively. Normally, they should not be set.
        """
        if force_insert and force_update:
            raise ValueError("Cannot force both insert and updating in model saving.")
        self.save_base(using=using, force_insert=force_insert, force_update=force_update)

    save.alters_data = True

    def save_base(self, raw=False, cls=None, origin=None, force_insert=False,
            force_update=False, using=None):
        """
        Does the heavy-lifting involved in saving. Subclasses shouldn't need to
        override this method. It's separate from save() in order to hide the
        need for overrides of save() to pass around internal-only parameters
        ('raw', 'cls', and 'origin').
        """
        using = using or router.db_for_write(self.__class__, instance=self)
        connection = connections[using]
        assert not (force_insert and force_update)
        if cls is None:
            cls = self.__class__
            meta = cls._meta
            if not meta.proxy:
                origin = cls
        else:
            meta = cls._meta

        if origin and not meta.auto_created:
            signals.pre_save.send(sender=origin, instance=self, raw=raw)

        # If we are in a raw save, save the object exactly as presented.
        # That means that we don't try to be smart about saving attributes
        # that might have come from the parent class - we just save the
        # attributes we have been given to the class we have been given.
        # We also go through this process to defer the save of proxy objects
        # to their actual underlying model.
        if not raw or meta.proxy:
            if meta.proxy:
                org = cls
            else:
                org = None
            for parent, field in meta.parents.items():
                # At this point, parent's primary key field may be unknown
                # (for example, from administration form which doesn't fill
                # this field). If so, fill it.
                if field and getattr(self, parent._meta.pk.attname) is None and getattr(self, field.attname) is not None:
                    setattr(self, parent._meta.pk.attname, getattr(self, field.attname))

                self.save_base(cls=parent, origin=org, using=using)

                if field:
                    setattr(self, field.attname, self._get_pk_val(parent._meta))
            if meta.proxy:
                return

        if not meta.proxy:
            non_pks = [f for f in meta.local_fields if not f.primary_key]

            # First, try an UPDATE. If that doesn't update anything, do an INSERT.
            pk_val = self._get_pk_val(meta)
            pk_set = pk_val is not None
            record_exists = True
            manager = cls._base_manager
            if pk_set:
                # Determine whether a record with the primary key already exists.
                if (force_update or (not force_insert and
                        manager.using(using).filter(pk=pk_val).exists())):
                    # It does already exist, so do an UPDATE.
                    if force_update or non_pks:
                        values = [(f, None, (raw and getattr(self, f.attname) or f.pre_save(self, False))) for f in non_pks]
                        rows = manager.using(using).filter(pk=pk_val)._update(values)
                        if force_update and not rows:
                            raise DatabaseError("Forced update did not affect any rows.")
                else:
                    record_exists = False
            if not pk_set or not record_exists:
                if meta.order_with_respect_to:
                    # If this is a model with an order_with_respect_to
                    # autopopulate the _order field
                    field = meta.order_with_respect_to
                    order_value = manager.using(using).filter(**{field.name: getattr(self, field.attname)}).count()
                    setattr(self, '_order', order_value)

                if not pk_set:
                    if force_update:
                        raise ValueError("Cannot force an update in save() with no primary key.")
                    values = [(f, f.get_db_prep_save(raw and getattr(self, f.attname) or f.pre_save(self, True), connection=connection))
                        for f in meta.local_fields if not isinstance(f, AutoField)]
                else:
                    values = [(f, f.get_db_prep_save(raw and getattr(self, f.attname) or f.pre_save(self, True), connection=connection))
                        for f in meta.local_fields]

                record_exists = False

                update_pk = bool(meta.has_auto_field and not pk_set)
                if values:
                    # Create a new record.
                    result = manager._insert(values, return_id=update_pk, using=using)
                else:
                    # Create a new record with defaults for everything.
                    result = manager._insert([(meta.pk, connection.ops.pk_default_value())], return_id=update_pk, raw_values=True, using=using)

                if update_pk:
                    setattr(self, meta.pk.attname, result)
            transaction.commit_unless_managed(using=using)

        # Store the database on which the object was saved
        self._state.db = using
        # Once saved, this is no longer a to-be-added instance.
        self._state.adding = False

        # Signal that the save is complete
        if origin and not meta.auto_created:
            signals.post_save.send(sender=origin, instance=self,
                created=(not record_exists), raw=raw)


    save_base.alters_data = True

    def _collect_sub_objects(self, seen_objs, parent=None, nullable=False):
        """
        Recursively populates seen_objs with all objects related to this
        object.

        When done, seen_objs.items() will be in the format:
            [(model_class, {pk_val: obj, pk_val: obj, ...}),
             (model_class, {pk_val: obj, pk_val: obj, ...}), ...]
        """
        pk_val = self._get_pk_val()
        if seen_objs.add(self.__class__, pk_val, self,
                         type(parent), parent, nullable):
            return

        for related in self._meta.get_all_related_objects():
            rel_opts_name = related.get_accessor_name()
            if not related.field.rel.multiple:
                try:
                    sub_obj = getattr(self, rel_opts_name)
                except ObjectDoesNotExist:
                    pass
                else:
                    sub_obj._collect_sub_objects(seen_objs, self, related.field.null)
            else:
                # To make sure we can access all elements, we can't use the
                # normal manager on the related object. So we work directly
                # with the descriptor object.
                for cls in self.__class__.mro():
                    if rel_opts_name in cls.__dict__:
                        rel_descriptor = cls.__dict__[rel_opts_name]
                        break
                else:
                    # in the case of a hidden fkey just skip it, it'll get
                    # processed as an m2m
                    if not related.field.rel.is_hidden():
                        raise AssertionError("Should never get here.")
                    else:
                        continue
                delete_qs = rel_descriptor.delete_manager(self).all()
                for sub_obj in delete_qs:
                    sub_obj._collect_sub_objects(seen_objs, self, related.field.null)

        for related in self._meta.get_all_related_many_to_many_objects():
            if related.field.rel.through:
                db = router.db_for_write(related.field.rel.through, instance=self)
                opts = related.field.rel.through._meta
                reverse_field_name = related.field.m2m_reverse_field_name()
                nullable = opts.get_field(reverse_field_name).null
                filters = {reverse_field_name: self}
                for sub_obj in related.field.rel.through._base_manager.using(db).filter(**filters):
                    sub_obj._collect_sub_objects(seen_objs, self, nullable)

        for f in self._meta.many_to_many:
            if f.rel.through:
                db = router.db_for_write(f.rel.through, instance=self)
                opts = f.rel.through._meta
                field_name = f.m2m_field_name()
                nullable = opts.get_field(field_name).null
                filters = {field_name: self}
                for sub_obj in f.rel.through._base_manager.using(db).filter(**filters):
                    sub_obj._collect_sub_objects(seen_objs, self, nullable)
            else:
                # m2m-ish but with no through table? GenericRelation: cascade delete
                for sub_obj in f.value_from_object(self).all():
                    # Generic relations not enforced by db constraints, thus we can set
                    # nullable=True, order does not matter
                    sub_obj._collect_sub_objects(seen_objs, self, True)

        # Handle any ancestors (for the model-inheritance case). We do this by
        # traversing to the most remote parent classes -- those with no parents
        # themselves -- and then adding those instances to the collection. That
        # will include all the child instances down to "self".
        parent_stack = [p for p in self._meta.parents.values() if p is not None]
        while parent_stack:
            link = parent_stack.pop()
            parent_obj = getattr(self, link.name)
            if parent_obj._meta.parents:
                parent_stack.extend(parent_obj._meta.parents.values())
                continue
            # At this point, parent_obj is base class (no ancestor models). So
            # delete it and all its descendents.
            parent_obj._collect_sub_objects(seen_objs)

    def delete(self, using=None):
        using = using or router.db_for_write(self.__class__, instance=self)
        assert self._get_pk_val() is not None, "%s object can't be deleted because its %s attribute is set to None." % (self._meta.object_name, self._meta.pk.attname)

        # Find all the objects than need to be deleted.
        seen_objs = CollectedObjects()
        self._collect_sub_objects(seen_objs)

        # Actually delete the objects.
        delete_objects(seen_objs, using)

    delete.alters_data = True

    def _get_FIELD_display(self, field):
        value = getattr(self, field.attname)
        return force_unicode(dict(field.flatchoices).get(value, value), strings_only=True)

    def _get_next_or_previous_by_FIELD(self, field, is_next, **kwargs):
        op = is_next and 'gt' or 'lt'
        order = not is_next and '-' or ''
        param = smart_str(getattr(self, field.attname))
        q = Q(**{'%s__%s' % (field.name, op): param})
        q = q|Q(**{field.name: param, 'pk__%s' % op: self.pk})
        qs = self.__class__._default_manager.using(self._state.db).filter(**kwargs).filter(q).order_by('%s%s' % (order, field.name), '%spk' % order)
        try:
            return qs[0]
        except IndexError:
            raise self.DoesNotExist("%s matching query does not exist." % self.__class__._meta.object_name)

    def _get_next_or_previous_in_order(self, is_next):
        cachename = "__%s_order_cache" % is_next
        if not hasattr(self, cachename):
            op = is_next and 'gt' or 'lt'
            order = not is_next and '-_order' or '_order'
            order_field = self._meta.order_with_respect_to
            obj = self._default_manager.filter(**{
                order_field.name: getattr(self, order_field.attname)
            }).filter(**{
                '_order__%s' % op: self._default_manager.values('_order').filter(**{
                    self._meta.pk.name: self.pk
                })
            }).order_by(order)[:1].get()
            setattr(self, cachename, obj)
        return getattr(self, cachename)

    def prepare_database_save(self, unused):
        return self.pk

    def clean(self):
        """
        Hook for doing any extra model-wide validation after clean() has been
        called on every field by self.clean_fields. Any ValidationError raised
        by this method will not be associated with a particular field; it will
        have a special-case association with the field defined by NON_FIELD_ERRORS.
        """
        pass

    def validate_unique(self, exclude=None):
        """
        Checks unique constraints on the model and raises ``ValidationError``
        if any failed.
        """
        unique_checks, date_checks = self._get_unique_checks(exclude=exclude)

        errors = self._perform_unique_checks(unique_checks)
        date_errors = self._perform_date_checks(date_checks)

        for k, v in date_errors.items():
            errors.setdefault(k, []).extend(v)

        if errors:
            raise ValidationError(errors)

    def _get_unique_checks(self, exclude=None):
        """
        Gather a list of checks to perform. Since validate_unique could be
        called from a ModelForm, some fields may have been excluded; we can't
        perform a unique check on a model that is missing fields involved
        in that check.
        Fields that did not validate should also be excluded, but they need
        to be passed in via the exclude argument.
        """
        if exclude is None:
            exclude = []
        unique_checks = []

        unique_togethers = [(self.__class__, self._meta.unique_together)]
        for parent_class in self._meta.parents.keys():
            if parent_class._meta.unique_together:
                unique_togethers.append((parent_class, parent_class._meta.unique_together))

        for model_class, unique_together in unique_togethers:
            for check in unique_together:
                for name in check:
                    # If this is an excluded field, don't add this check.
                    if name in exclude:
                        break
                else:
                    unique_checks.append((model_class, tuple(check)))

        # These are checks for the unique_for_<date/year/month>.
        date_checks = []

        # Gather a list of checks for fields declared as unique and add them to
        # the list of checks.

        fields_with_class = [(self.__class__, self._meta.local_fields)]
        for parent_class in self._meta.parents.keys():
            fields_with_class.append((parent_class, parent_class._meta.local_fields))

        for model_class, fields in fields_with_class:
            for f in fields:
                name = f.name
                if name in exclude:
                    continue
                if f.unique:
                    unique_checks.append((model_class, (name,)))
                if f.unique_for_date and f.unique_for_date not in exclude:
                    date_checks.append((model_class, 'date', name, f.unique_for_date))
                if f.unique_for_year and f.unique_for_year not in exclude:
                    date_checks.append((model_class, 'year', name, f.unique_for_year))
                if f.unique_for_month and f.unique_for_month not in exclude:
                    date_checks.append((model_class, 'month', name, f.unique_for_month))
        return unique_checks, date_checks

    def _perform_unique_checks(self, unique_checks):
        errors = {}

        for model_class, unique_check in unique_checks:
            # Try to look up an existing object with the same values as this
            # object's values for all the unique field.

            lookup_kwargs = {}
            for field_name in unique_check:
                f = self._meta.get_field(field_name)
                lookup_value = getattr(self, f.attname)
                if lookup_value is None:
                    # no value, skip the lookup
                    continue
                if f.primary_key and not self._state.adding:
                    # no need to check for unique primary key when editing
                    continue
                lookup_kwargs[str(field_name)] = lookup_value

            # some fields were skipped, no reason to do the check
            if len(unique_check) != len(lookup_kwargs.keys()):
                continue

            qs = model_class._default_manager.filter(**lookup_kwargs)

            # Exclude the current object from the query if we are editing an
            # instance (as opposed to creating a new one)
            if not self._state.adding and self.pk is not None:
                qs = qs.exclude(pk=self.pk)

            if qs.exists():
                if len(unique_check) == 1:
                    key = unique_check[0]
                else:
                    key = NON_FIELD_ERRORS
                errors.setdefault(key, []).append(self.unique_error_message(model_class, unique_check))

        return errors

    def _perform_date_checks(self, date_checks):
        errors = {}
        for model_class, lookup_type, field, unique_for in date_checks:
            lookup_kwargs = {}
            # there's a ticket to add a date lookup, we can remove this special
            # case if that makes it's way in
            date = getattr(self, unique_for)
            if date is None:
                continue
            if lookup_type == 'date':
                lookup_kwargs['%s__day' % unique_for] = date.day
                lookup_kwargs['%s__month' % unique_for] = date.month
                lookup_kwargs['%s__year' % unique_for] = date.year
            else:
                lookup_kwargs['%s__%s' % (unique_for, lookup_type)] = getattr(date, lookup_type)
            lookup_kwargs[field] = getattr(self, field)

            qs = model_class._default_manager.filter(**lookup_kwargs)
            # Exclude the current object from the query if we are editing an
            # instance (as opposed to creating a new one)
            if not self._state.adding and self.pk is not None:
                qs = qs.exclude(pk=self.pk)

            if qs.exists():
                errors.setdefault(field, []).append(
                    self.date_error_message(lookup_type, field, unique_for)
                )
        return errors

    def date_error_message(self, lookup_type, field, unique_for):
        opts = self._meta
        return _(u"%(field_name)s must be unique for %(date_field)s %(lookup)s.") % {
            'field_name': unicode(capfirst(opts.get_field(field).verbose_name)),
            'date_field': unicode(capfirst(opts.get_field(unique_for).verbose_name)),
            'lookup': lookup_type,
        }

    def unique_error_message(self, model_class, unique_check):
        opts = model_class._meta
        model_name = capfirst(opts.verbose_name)

        # A unique field
        if len(unique_check) == 1:
            field_name = unique_check[0]
            field_label = capfirst(opts.get_field(field_name).verbose_name)
            # Insert the error into the error dict, very sneaky
            return _(u"%(model_name)s with this %(field_label)s already exists.") %  {
                'model_name': unicode(model_name),
                'field_label': unicode(field_label)
            }
        # unique_together
        else:
            field_labels = map(lambda f: capfirst(opts.get_field(f).verbose_name), unique_check)
            field_labels = get_text_list(field_labels, _('and'))
            return _(u"%(model_name)s with this %(field_label)s already exists.") %  {
                'model_name': unicode(model_name),
                'field_label': unicode(field_labels)
            }

    def full_clean(self, exclude=None):
        """
        Calls clean_fields, clean, and validate_unique, on the model,
        and raises a ``ValidationError`` for any errors that occured.
        """
        errors = {}
        if exclude is None:
            exclude = []

        try:
            self.clean_fields(exclude=exclude)
        except ValidationError, e:
            errors = e.update_error_dict(errors)

        # Form.clean() is run even if other validation fails, so do the
        # same with Model.clean() for consistency.
        try:
            self.clean()
        except ValidationError, e:
            errors = e.update_error_dict(errors)

        # Run unique checks, but only for fields that passed validation.
        for name in errors.keys():
            if name != NON_FIELD_ERRORS and name not in exclude:
                exclude.append(name)
        try:
            self.validate_unique(exclude=exclude)
        except ValidationError, e:
            errors = e.update_error_dict(errors)

        if errors:
            raise ValidationError(errors)

    def clean_fields(self, exclude=None):
        """
        Cleans all fields and raises a ValidationError containing message_dict
        of all validation errors if any occur.
        """
        if exclude is None:
            exclude = []

        errors = {}
        for f in self._meta.fields:
            if f.name in exclude:
                continue
            # Skip validation for empty fields with blank=True. The developer
            # is responsible for making sure they have a valid value.
            raw_value = getattr(self, f.attname)
            if f.blank and raw_value in validators.EMPTY_VALUES:
                continue
            try:
                setattr(self, f.attname, f.clean(raw_value, self))
            except ValidationError, e:
                errors[f.name] = e.messages

        if errors:
            raise ValidationError(errors)


############################################
# HELPER FUNCTIONS (CURRIED MODEL METHODS) #
############################################

# ORDERING METHODS #########################

def method_set_order(ordered_obj, self, id_list, using=None):
    if using is None:
        using = DEFAULT_DB_ALIAS
    rel_val = getattr(self, ordered_obj._meta.order_with_respect_to.rel.field_name)
    order_name = ordered_obj._meta.order_with_respect_to.name
    # FIXME: It would be nice if there was an "update many" version of update
    # for situations like this.
    for i, j in enumerate(id_list):
        ordered_obj.objects.filter(**{'pk': j, order_name: rel_val}).update(_order=i)
    transaction.commit_unless_managed(using=using)


def method_get_order(ordered_obj, self):
    rel_val = getattr(self, ordered_obj._meta.order_with_respect_to.rel.field_name)
    order_name = ordered_obj._meta.order_with_respect_to.name
    pk_name = ordered_obj._meta.pk.name
    return [r[pk_name] for r in
            ordered_obj.objects.filter(**{order_name: rel_val}).values(pk_name)]


##############################################
# HELPER FUNCTIONS (CURRIED MODEL FUNCTIONS) #
##############################################

def get_absolute_url(opts, func, self, *args, **kwargs):
    return settings.ABSOLUTE_URL_OVERRIDES.get('%s.%s' % (opts.app_label, opts.module_name), func)(self, *args, **kwargs)


########
# MISC #
########

class Empty(object):
    pass

def simple_class_factory(model, attrs):
    """Used to unpickle Models without deferred fields.

    We need to do this the hard way, rather than just using
    the default __reduce__ implementation, because of a
    __deepcopy__ problem in Python 2.4
    """
    return model

def model_unpickle(model, attrs, factory):
    """
    Used to unpickle Model subclasses with deferred fields.
    """
    cls = factory(model, attrs)
    return cls.__new__(cls)
model_unpickle.__safe_for_unpickle__ = True

if sys.version_info < (2, 5):
    # Prior to Python 2.5, Exception was an old-style class
    def subclass_exception(name, parents, unused):
        return types.ClassType(name, parents, {})
else:
    def subclass_exception(name, parents, module):
        return type(name, parents, {'__module__': module})
