- add a new example section for "join conditions", start putting

the primaryjoin examples there

4 files changed, 217 insertions, 0 deletions

diff --git a/doc/build/orm/examples.rst b/doc/build/orm/examples.rst
index 484865e7e..99ca4bb8d 100644
--- a/doc/build/orm/examples.rst
+++ b/doc/build/orm/examples.rst
@@ -57,6 +57,13 @@ Nested Sets
 
 .. automodule:: examples.nested_sets
 
+.. _examples_relationships:
+
+Relationship Join Conditions
+----------------------------
+
+.. automodule:: examples.join_conditions
+
 .. _examples_xmlpersistence:
 
 XML Persistence
diff --git a/examples/join_conditions/__init__.py b/examples/join_conditions/__init__.py
new file mode 100644
index 000000000..3a561d084
--- /dev/null
+++ b/examples/join_conditions/__init__.py
@@ -0,0 +1,7 @@
+"""Examples of various :func:`.orm.relationship` configurations,
+which make use of the ``primaryjoin`` argument to compose special types
+of join conditions.
+
+.. autosource::
+
+"""
\ No newline at end of file
diff --git a/examples/join_conditions/cast.py b/examples/join_conditions/cast.py
new file mode 100644
index 000000000..246bc1d57
--- /dev/null
+++ b/examples/join_conditions/cast.py
@@ -0,0 +1,95 @@
+"""Illustrate a :func:`.relationship` that joins two columns where those
+columns are not of the same type, and a CAST must be used on the SQL
+side in order to match them.
+
+When complete, we'd like to see a load of the relationship to look like::
+
+    -- load the primary row, a_id is a string
+    SELECT a.id AS a_id_1, a.a_id AS a_a_id
+    FROM a
+    WHERE a.a_id = '2'
+
+    -- then load the collection using CAST, b.a_id is an integer
+    SELECT b.id AS b_id, b.a_id AS b_a_id
+    FROM b
+    WHERE CAST('2' AS INTEGER) = b.a_id
+
+The relationship is essentially configured as follows::
+
+    class B(Base):
+        # ...
+
+        a = relationship(A,
+                    primaryjoin=cast(A.a_id, Integer) == foreign(B.a_id),
+                    backref="bs")
+
+Where above, we are making use of the :func:`.cast` function in order
+to produce CAST, as well as the :func:`.foreign` :term:`annotation` function
+in order to note to the ORM that ``B.a_id`` should be treated like the
+"foreign key" column.
+
+"""
+from sqlalchemy import *
+from sqlalchemy.orm import *
+from sqlalchemy.ext.declarative import declarative_base
+
+Base = declarative_base()
+
+class StringAsInt(TypeDecorator):
+    """Coerce string->integer type.
+
+    This is needed only if the relationship() from
+    int to string is writable, as SQLAlchemy will copy
+    the string parent values into the integer attribute
+    on the child during a flush.
+
+    """
+    impl = Integer
+    def process_bind_param(self, value, dialect):
+        if value is not None:
+            value = int(value)
+        return value
+
+class A(Base):
+    """Parent. The referenced column is a string type."""
+
+    __tablename__ = 'a'
+
+    id = Column(Integer, primary_key=True)
+    a_id = Column(String)
+
+class B(Base):
+    """Child.  The column we reference 'A' with is an integer."""
+
+    __tablename__ = 'b'
+
+    id = Column(Integer, primary_key=True)
+    a_id = Column(StringAsInt)
+    a = relationship("A",
+                # specify primaryjoin.  The string form is optional
+                # here, but note that Declarative makes available all
+                # of the built-in functions we might need, including
+                # cast() and foreign().
+                primaryjoin="cast(A.a_id, Integer) == foreign(B.a_id)",
+                backref="bs")
+
+# we demonstrate with SQLite, but the important part
+# is the CAST rendered in the SQL output.
+
+e = create_engine('sqlite://', echo=True)
+Base.metadata.create_all(e)
+
+s = Session(e)
+
+s.add_all([
+    A(a_id="1"),
+    A(a_id="2", bs=[B(), B()]),
+    A(a_id="3", bs=[B()]),
+])
+s.commit()
+
+b1 = s.query(B).filter_by(a_id="2").first()
+print(b1.a)
+
+a1 = s.query(A).filter_by(a_id="2").first()
+print(a1.bs)
\ No newline at end of file
diff --git a/examples/join_conditions/threeway.py b/examples/join_conditions/threeway.py
new file mode 100644
index 000000000..13df0f349
--- /dev/null
+++ b/examples/join_conditions/threeway.py
@@ -0,0 +1,108 @@
+"""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)