# SQLAlchemy 0.9 Documentation

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Release: 0.9.3 | Release Date: February 19, 2014

# Source code for examples.sharding.attribute_shard

    # step 1. imports
    from sqlalchemy import (create_engine, MetaData, Table, Column, Integer,
        String, ForeignKey, Float, DateTime, event)
    from sqlalchemy.orm import sessionmaker, mapper, relationship
    from sqlalchemy.ext.horizontal_shard import ShardedSession
    from sqlalchemy.sql import operators, visitors

    import datetime

    # step 2. databases.
    # db1 is used for id generation. The "pool_threadlocal"
    # causes the id_generator() to use the same connection as that
    # of an ongoing transaction within db1.
    echo = True
    db1 = create_engine('sqlite://', echo=echo, pool_threadlocal=True)
    db2 = create_engine('sqlite://', echo=echo)
    db3 = create_engine('sqlite://', echo=echo)
    db4 = create_engine('sqlite://', echo=echo)

    # step 3. create session function.  this binds the shard ids
    # to databases within a ShardedSession and returns it.
    create_session = sessionmaker(class_=ShardedSession)

    create_session.configure(shards={
        'north_america':db1,
        'asia':db2,
        'europe':db3,
        'south_america':db4
    })

    # step 4.  table setup.
    meta = MetaData()

    # we need a way to create identifiers which are unique across all
    # databases.  one easy way would be to just use a composite primary key, where one
    # value is the shard id.  but here, we'll show something more "generic", an
    # id generation function.  we'll use a simplistic "id table" stored in database
    # #1.  Any other method will do just as well; UUID, hilo, application-specific, etc.

    ids = Table('ids', meta,
        Column('nextid', Integer, nullable=False))

    def id_generator(ctx):
        # in reality, might want to use a separate transaction for this.
        c = db1.connect()
        nextid = c.execute(ids.select(for_update=True)).scalar()
        c.execute(ids.update(values={ids.c.nextid : ids.c.nextid + 1}))
        return nextid

    # table setup.  we'll store a lead table of continents/cities,
    # and a secondary table storing locations.
    # a particular row will be placed in the database whose shard id corresponds to the
    # 'continent'.  in this setup, secondary rows in 'weather_reports' will
    # be placed in the same DB as that of the parent, but this can be changed
    # if you're willing to write more complex sharding functions.

    weather_locations = Table("weather_locations", meta,
            Column('id', Integer, primary_key=True, default=id_generator),
            Column('continent', String(30), nullable=False),
            Column('city', String(50), nullable=False)
        )

    weather_reports = Table("weather_reports", meta,
        Column('id', Integer, primary_key=True),
        Column('location_id', Integer, ForeignKey('weather_locations.id')),
        Column('temperature', Float),
        Column('report_time', DateTime, default=datetime.datetime.now),
    )

    # create tables
    for db in (db1, db2, db3, db4):
        meta.drop_all(db)
        meta.create_all(db)

    # establish initial "id" in db1
    db1.execute(ids.insert(), nextid=1)

    # step 5. define sharding functions.

    # we'll use a straight mapping of a particular set of "country"
    # attributes to shard id.
    shard_lookup = {
        'North America':'north_america',
        'Asia':'asia',
        'Europe':'europe',
        'South America':'south_america'
    }

    def shard_chooser(mapper, instance, clause=None):
        """shard chooser.

        looks at the given instance and returns a shard id
        note that we need to define conditions for
        the WeatherLocation class, as well as our secondary Report class which will
        point back to its WeatherLocation via its 'location' attribute.

        """
        if isinstance(instance, WeatherLocation):
            return shard_lookup[instance.continent]
        else:
            return shard_chooser(mapper, instance.location)

    def id_chooser(query, ident):
        """id chooser.

        given a primary key, returns a list of shards
        to search.  here, we don't have any particular information from a
        pk so we just return all shard ids. often, youd want to do some
        kind of round-robin strategy here so that requests are evenly
        distributed among DBs.

        """
        return ['north_america', 'asia', 'europe', 'south_america']

    def query_chooser(query):
        """query chooser.

        this also returns a list of shard ids, which can
        just be all of them.  but here we'll search into the Query in order
        to try to narrow down the list of shards to query.

        """
        ids = []

        # we'll grab continent names as we find them
        # and convert to shard ids
        for column, operator, value in _get_query_comparisons(query):
            # "shares_lineage()" returns True if both columns refer to the same
            # statement column, adjusting for any annotations present.
            # (an annotation is an internal clone of a Column object
            # and occur when using ORM-mapped attributes like
            # "WeatherLocation.continent"). A simpler comparison, though less accurate,
            # would be "column.key == 'continent'".
            if column.shares_lineage(weather_locations.c.continent):
                if operator == operators.eq:
                    ids.append(shard_lookup[value])
                elif operator == operators.in_op:
                    ids.extend(shard_lookup[v] for v in value)

        if len(ids) == 0:
            return ['north_america', 'asia', 'europe', 'south_america']
        else:
            return ids

    def _get_query_comparisons(query):
        """Search an orm.Query object for binary expressions.

        Returns expressions which match a Column against one or more
        literal values as a list of tuples of the form
        (column, operator, values).   "values" is a single value
        or tuple of values depending on the operator.

        """
        binds = {}
        clauses = set()
        comparisons = []

        def visit_bindparam(bind):
            # visit a bind parameter.

            # check in _params for it first
            if bind.key in query._params:
                value = query._params[bind.key]
            elif bind.callable:
                # some ORM functions (lazy loading)
                # place the bind's value as a
                # callable for deferred evaulation.
                value = bind.callable()
            else:
                # just use .value
                value = bind.value

            binds[bind] = value

        def visit_column(column):
            clauses.add(column)

        def visit_binary(binary):
            # special handling for "col IN (params)"
            if binary.left in clauses and \
                    binary.operator == operators.in_op and \
                    hasattr(binary.right, 'clauses'):
                comparisons.append(
                    (binary.left, binary.operator,
                        tuple(binds[bind] for bind in binary.right.clauses)
                    )
                )
            elif binary.left in clauses and binary.right in binds:
                comparisons.append(
                    (binary.left, binary.operator,binds[binary.right])
                )

            elif binary.left in binds and binary.right in clauses:
                comparisons.append(
                    (binary.right, binary.operator,binds[binary.left])
                )

        # here we will traverse through the query's criterion, searching
        # for SQL constructs.  We will place simple column comparisons
        # into a list.
        if query._criterion is not None:
            visitors.traverse_depthfirst(query._criterion, {},
                        {'bindparam':visit_bindparam,
                            'binary':visit_binary,
                            'column':visit_column
                        }
            )
        return comparisons

    # further configure create_session to use these functions
    create_session.configure(
                        shard_chooser=shard_chooser,
                        id_chooser=id_chooser,
                        query_chooser=query_chooser
                        )

    # step 6.  mapped classes.
    class WeatherLocation(object):
        def __init__(self, continent, city):
            self.continent = continent
            self.city = city

    class Report(object):
        def __init__(self, temperature):
            self.temperature = temperature

    # step 7.  mappers
    mapper(WeatherLocation, weather_locations, properties={
        'reports':relationship(Report, backref='location')
    })

    mapper(Report, weather_reports)

    # step 8 (optional), events.  The "shard_id" is placed
    # in the QueryContext where it can be intercepted and associated
    # with objects, if needed.

    def add_shard_id(instance, ctx):
        instance.shard_id = ctx.attributes["shard_id"]

    event.listen(WeatherLocation, "load", add_shard_id)
    event.listen(Report, "load", add_shard_id)

    # save and load objects!

    tokyo = WeatherLocation('Asia', 'Tokyo')
    newyork = WeatherLocation('North America', 'New York')
    toronto = WeatherLocation('North America', 'Toronto')
    london = WeatherLocation('Europe', 'London')
    dublin = WeatherLocation('Europe', 'Dublin')
    brasilia = WeatherLocation('South America', 'Brasila')
    quito = WeatherLocation('South America', 'Quito')

    tokyo.reports.append(Report(80.0))
    newyork.reports.append(Report(75))
    quito.reports.append(Report(85))

    sess = create_session()
    for c in [tokyo, newyork, toronto, london, dublin, brasilia, quito]:
        sess.add(c)
    sess.commit()

    tokyo_id = tokyo.id

    sess.close()

    t = sess.query(WeatherLocation).get(tokyo_id)
    assert t.city == tokyo.city
    assert t.reports[0].temperature == 80.0

    north_american_cities = sess.query(WeatherLocation).filter(WeatherLocation.continent == 'North America')
    assert [c.city for c in north_american_cities] == ['New York', 'Toronto']

    asia_and_europe = sess.query(WeatherLocation).filter(WeatherLocation.continent.in_(['Europe', 'Asia']))
    assert set([c.city for c in asia_and_europe]) == set(['Tokyo', 'London', 'Dublin'])