# SQLAlchemy 0.9 Documentation

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Release: 0.9.3 | Release Date: February 19, 2014

# Source code for examples.versioned_history.history_meta

    """Versioned mixin class and other utilities."""

    from sqlalchemy.ext.declarative import declared_attr
    from sqlalchemy.orm import mapper, attributes, object_mapper
    from sqlalchemy.orm.exc import UnmappedColumnError
    from sqlalchemy import Table, Column, ForeignKeyConstraint, Integer, DateTime
    from sqlalchemy import event
    import datetime
    from sqlalchemy.orm.properties import RelationshipProperty

    def col_references_table(col, table):
        for fk in col.foreign_keys:
            if fk.references(table):
                return True
        return False

    def _is_versioning_col(col):
        return "version_meta" in col.info

    def _history_mapper(local_mapper):
        cls = local_mapper.class_

        # set the "active_history" flag
        # on on column-mapped attributes so that the old version
        # of the info is always loaded (currently sets it on all attributes)
        for prop in local_mapper.iterate_properties:
            getattr(local_mapper.class_, prop.key).impl.active_history = True

        super_mapper = local_mapper.inherits
        super_history_mapper = getattr(cls, '__history_mapper__', None)

        polymorphic_on = None
        super_fks = []

        if not super_mapper or local_mapper.local_table is not super_mapper.local_table:
            cols = []
            for column in local_mapper.local_table.c:
                if _is_versioning_col(column):
                    continue

                col = column.copy()
                col.unique = False

                if super_mapper and col_references_table(column, super_mapper.local_table):
                    super_fks.append((col.key, list(super_history_mapper.local_table.primary_key)[0]))

                cols.append(col)

                if column is local_mapper.polymorphic_on:
                    polymorphic_on = col

            if super_mapper:
                super_fks.append(('version', super_history_mapper.local_table.c.version))

            version_meta = {"version_meta": True}  # add column.info to identify
                                                   # columns specific to versioning

            # "version" stores the integer version id.  This column is
            # required.
            cols.append(Column('version', Integer, primary_key=True,
                                autoincrement=False, info=version_meta))

            # "changed" column stores the UTC timestamp of when the
            # history row was created.
            # This column is optional and can be omitted.
            cols.append(Column('changed', DateTime,
                                default=datetime.datetime.utcnow,
                                info=version_meta))

            if super_fks:
                cols.append(ForeignKeyConstraint(*zip(*super_fks)))

            table = Table(local_mapper.local_table.name + '_history',
                            local_mapper.local_table.metadata,
                            *cols,
                            schema=local_mapper.local_table.schema
            )
        else:
            # single table inheritance.  take any additional columns that may have
            # been added and add them to the history table.
            for column in local_mapper.local_table.c:
                if column.key not in super_history_mapper.local_table.c:
                    col = column.copy()
                    col.unique = False
                    super_history_mapper.local_table.append_column(col)
            table = None

        if super_history_mapper:
            bases = (super_history_mapper.class_,)
        else:
            bases = local_mapper.base_mapper.class_.__bases__
        versioned_cls = type.__new__(type, "%sHistory" % cls.__name__, bases, {})

        m = mapper(
                versioned_cls,
                table,
                inherits=super_history_mapper,
                polymorphic_on=polymorphic_on,
                polymorphic_identity=local_mapper.polymorphic_identity
                )
        cls.__history_mapper__ = m

        if not super_history_mapper:
            local_mapper.local_table.append_column(
                Column('version', Integer, default=1, nullable=False)
            )
            local_mapper.add_property("version", local_mapper.local_table.c.version)

    class Versioned(object):
        @declared_attr
        def __mapper_cls__(cls):
            def map(cls, *arg, **kw):
                mp = mapper(cls, *arg, **kw)
                _history_mapper(mp)
                return mp
            return map

    def versioned_objects(iter):
        for obj in iter:
            if hasattr(obj, '__history_mapper__'):
                yield obj

    def create_version(obj, session, deleted=False):
        obj_mapper = object_mapper(obj)
        history_mapper = obj.__history_mapper__
        history_cls = history_mapper.class_

        obj_state = attributes.instance_state(obj)

        attr = {}

        obj_changed = False

        for om, hm in zip(obj_mapper.iterate_to_root(), history_mapper.iterate_to_root()):
            if hm.single:
                continue

            for hist_col in hm.local_table.c:
                if _is_versioning_col(hist_col):
                    continue

                obj_col = om.local_table.c[hist_col.key]

                # get the value of the
                # attribute based on the MapperProperty related to the
                # mapped column.  this will allow usage of MapperProperties
                # that have a different keyname than that of the mapped column.
                try:
                    prop = obj_mapper.get_property_by_column(obj_col)
                except UnmappedColumnError:
                    # in the case of single table inheritance, there may be
                    # columns on the mapped table intended for the subclass only.
                    # the "unmapped" status of the subclass column on the
                    # base class is a feature of the declarative module as of sqla 0.5.2.
                    continue

                # expired object attributes and also deferred cols might not be in the
                # dict.  force it to load no matter what by using getattr().
                if prop.key not in obj_state.dict:
                    getattr(obj, prop.key)

                a, u, d = attributes.get_history(obj, prop.key)

                if d:
                    attr[hist_col.key] = d[0]
                    obj_changed = True
                elif u:
                    attr[hist_col.key] = u[0]
                else:
                    # if the attribute had no value.
                    attr[hist_col.key] = a[0]
                    obj_changed = True

        if not obj_changed:
            # not changed, but we have relationships.  OK
            # check those too
            for prop in obj_mapper.iterate_properties:
                if isinstance(prop, RelationshipProperty) and \
                    attributes.get_history(obj, prop.key,
                            passive=attributes.PASSIVE_NO_INITIALIZE).has_changes():
                    for p in prop.local_columns:
                        if p.foreign_keys:
                            obj_changed = True
                            break
                    if obj_changed is True:
                        break

        if not obj_changed and not deleted:
            return

        attr['version'] = obj.version
        hist = history_cls()
        for key, value in attr.items():
            setattr(hist, key, value)
        session.add(hist)
        obj.version += 1

    def versioned_session(session):
        @event.listens_for(session, 'before_flush')
        def before_flush(session, flush_context, instances):
            for obj in versioned_objects(session.dirty):
                create_version(obj, session)
            for obj in versioned_objects(session.deleted):
                create_version(obj, session, deleted=True)