SQLAlchemy: Relation table with composite primary key

问题

I have a set of tables that look like:

workflows = Table('workflows', Base.metadata,
                  Column('id', Integer, primary_key=True),
                 )

actions = Table('actions', Base.metadata,
                Column('name', String, primary_key=True),
                Column('workflow_id', Integer, ForeignKey(workflows.c.id), primary_key=True),
               )

action_dependencies = Table('action_dependencies', Base.metadata,
                            Column('workflow_id', Integer, ForeignKey(workflows.c.id), primary_key=True),
                            Column('parent_action', String, ForeignKey(actions.c.name), primary_key=True),
                            Column('child_action', String, ForeignKey(actions.c.name), primary_key=True),
                           )

My ORM classes look like:

class Workflow(Base):
    __table__ = workflows

    actions = relationship("Action", order_by="Action.name", backref="workflow")


class Action(Base):
    __table__ = actions

    children = relationship("Action",
                            secondary=action_dependencies,
                            primaryjoin=actions.c.name == action_dependencies.c.parent_action,
                            secondaryjoin=actions.c.name == action_dependencies.c.child_action,
                            backref="parents"
                           )

So in my system, each action is uniquely identified by a combination of a workflow id and its name. I'd like each action to have parents and children attribute that refers its parent and child actions. Each action can have multiple parents and children.

The problem occurs when I have a function such as :

def set_parents(session, workflow_id, action_name, parents):
    action = session.query(db.Action).filter(db.Action.workflow_id == workflow.id).filter(db.Action.name == action_name).one()

    for parent_name in parents:
        parent = session.query(db.Action).filter(db.Action.workflow_id == workflow.id).filter(db.Action.name == parent_name).one()
        action.parents.append(parent)

    session.commit()

I get an error like:

IntegrityError: (IntegrityError) action_dependencies.workflow_id may not be NULL u'INSERT INTO action_dependencies (parent_action, child_action) VALUES (?, ?)' (u'directory_creator', u'packing')

How do I get the relationship to set the workflow_id correctly?

回答1:

See below working code. The key points are those I mentioned in the comments:

proper composite ForeignKeys
correct relationship configuration using the FKs

Code:

workflows = Table('workflows', Base.metadata,
                  Column('id', Integer, primary_key=True),
                 )

actions = Table('actions', Base.metadata,
                Column('workflow_id', Integer, ForeignKey(workflows.c.id), primary_key=True),
                Column('name', String, primary_key=True),
               )

action_dependencies = Table('action_dependencies', Base.metadata,
                            Column('workflow_id', Integer, ForeignKey(workflows.c.id), primary_key=True),
                            Column('parent_action', String, ForeignKey(actions.c.name), primary_key=True),
                            Column('child_action', String, ForeignKey(actions.c.name), primary_key=True),
                            ForeignKeyConstraint(['workflow_id', 'parent_action'], ['actions.workflow_id', 'actions.name']),
                            ForeignKeyConstraint(['workflow_id', 'child_action'], ['actions.workflow_id', 'actions.name']),
                           )
class Workflow(Base):
    __table__ = workflows
    actions = relationship("Action", order_by="Action.name", backref="workflow")

class Action(Base):
    __table__ = actions
    children = relationship("Action",
                            secondary=action_dependencies,
                            primaryjoin=and_(actions.c.name == action_dependencies.c.parent_action,
                                actions.c.workflow_id == action_dependencies.c.workflow_id),
                            secondaryjoin=and_(actions.c.name == action_dependencies.c.child_action,
                                actions.c.workflow_id == action_dependencies.c.workflow_id),
                            backref="parents"
                           )

# create db schema
Base.metadata.create_all(engine)

# create entities
w_1 = Workflow()
w_2 = Workflow()
a_11 = Action(name="ac-11", workflow=w_1)
a_12 = Action(name="ac-12", workflow=w_1)
a_21 = Action(name="ac-21", workflow=w_2)
a_22 = Action(name="ac-22", workflow=w_2)
session.add(w_1)
session.add(w_2)
a_22.parents.append(a_21)
session.commit()
session.expunge_all()
print '-'*80

# helper functions
def get_workflow(id):
    return session.query(Workflow).get(id)
def get_action(name):
    return session.query(Action).filter_by(name=name).one()

# test another OK
a_11 = get_action("ac-11")
a_12 = get_action("ac-12")
a_11.children.append(a_12)
session.commit()
session.expunge_all()
print '-'*80

# test KO (THIS SHOULD FAIL VIOLATING FK-constraint)
a_11 = get_action("ac-11")
a_22 = get_action("ac-22")
a_11.children.append(a_22)
session.commit()
session.expunge_all()
print '-'*80

回答2:

I don't think it's correct to have the primary key a foreign key. How does that work?

But to make composite constraint, a key that is "unique together", use this in the table definition:

UniqueConstraint(u"name", u"workflow_id"),

But if you really want it to be the primary key also you can use this:

PrimaryKeyConstraint(*columns, **kw)

http://docs.sqlalchemy.org/en/latest/core/schema.html#sqlalchemy.schema.PrimaryKeyConstraint

来源：https://stackoverflow.com/questions/10525797/sqlalchemy-relation-table-with-composite-primary-key

标签

python

orm

sqlalchemy

foreign-key-relationship