SQLAlchemy 0.9 Documentation

Release: 0.9.4 | Release Date: March 28, 2014 | Download PDF
SQLAlchemy 0.9 Documentation » Module code » examples.join_conditions.threeway

examples.join_conditions.threeway

Source code for examples.join_conditions.threeway

"""Illustrate a "three way join" - where a primary table joins to a remote
table via an association table, but then the primary table also needs
to refer to some columns in the remote table directly.

E.g.::

    first.first_id      -> second.first_id
                           second.other_id --> partitioned.other_id
    first.partition_key ---------------------> partitioned.partition_key

For a relationship like this, "second" is a lot like a "secondary" table,
but the mechanics aren't present within the "secondary" feature to allow
for the join directly between first and partitioned.  Instead, we
will derive a selectable from partitioned and second combined together, then
link first to that derived selectable.

If we define the derived selectable as::

    second JOIN partitioned ON second.other_id = partitioned.other_id

A JOIN from first to this derived selectable is then::

    first JOIN (second JOIN partitioned
                ON second.other_id = partitioned.other_id)
          ON first.first_id = second.first_id AND
             first.partition_key = partitioned.partition_key

We will use the "non primary mapper" feature in order to produce this.
A non primary mapper is essentially an "extra" :func:`.mapper` that we can
use to associate a particular class with some selectable that is
not its usual mapped table.   It is used only when called upon within
a Query (or a :func:`.relationship`).


"""
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class First(Base):
    __tablename__ = 'first'

    first_id = Column(Integer, primary_key=True)
    partition_key = Column(String)

    def __repr__(self):
        return ("First(%s, %s)" % (self.first_id, self.partition_key))

class Second(Base):
    __tablename__ = 'second'

    first_id = Column(Integer, primary_key=True)
    other_id = Column(Integer, primary_key=True)

class Partitioned(Base):
    __tablename__ = 'partitioned'

    other_id = Column(Integer, primary_key=True)
    partition_key = Column(String, primary_key=True)

    def __repr__(self):
        return ("Partitioned(%s, %s)" % (self.other_id, self.partition_key))


j = join(Partitioned, Second, Partitioned.other_id == Second.other_id)

partitioned_second = mapper(Partitioned, j, non_primary=True, properties={
        # note we need to disambiguate columns here - the join()
        # will provide them as j.c.<tablename>_<colname> for access,
        # but they retain their real names in the mapping
        "other_id": [j.c.partitioned_other_id, j.c.second_other_id],
    })

First.partitioned = relationship(
                            partitioned_second,
                            primaryjoin=and_(
                                First.partition_key == partitioned_second.c.partition_key,
                                First.first_id == foreign(partitioned_second.c.first_id)
                            ), innerjoin=True)

# when using any database other than SQLite, we will get a nested
# join, e.g. "first JOIN (partitioned JOIN second ON ..) ON ..".
# On SQLite, SQLAlchemy needs to render a full subquery.
e = create_engine("sqlite://", echo=True)

Base.metadata.create_all(e)
s = Session(e)
s.add_all([
    First(first_id=1, partition_key='p1'),
    First(first_id=2, partition_key='p1'),
    First(first_id=3, partition_key='p2'),
    Second(first_id=1, other_id=1),
    Second(first_id=2, other_id=1),
    Second(first_id=3, other_id=2),
    Partitioned(partition_key='p1', other_id=1),
    Partitioned(partition_key='p1', other_id=2),
    Partitioned(partition_key='p2', other_id=2),
])
s.commit()

for row in s.query(First, Partitioned).join(First.partitioned):
    print(row)

for f in s.query(First):
    for p in f.partitioned:
        print(f.partition_key, p.partition_key)