How can I cascade delete from a many to one only when deleting the many
means this one has no other children?
Imagine the following mapping:
class Container(Base):
__tablename__ = 'container'
pk = sa.Column(sa.Integer, primary_key=True)
entries = relationship('Entry', backref='container', lazy='dynamic',
cascade="all, delete, delete-orphan")
class Entry(Base):
__tablename__ = 'entry'
pk = sa.Column(sa.Integer, primary_key=True)
container_pk = sa.Column(sa.Integer, sa.ForeignKey('container.pk'),
tag_pk = sa.Column(sa.Integer, sa.ForeignKey('tag.pk'))
class Tag(Base):
__tablename__ = 'tag'
pk = sa.Column(sa.Integer, primary_key=True)
description = sa.Column(sa.String, nullable=False)
entries = relationship("Entry", backref='tag')
A Container has zero or more Entry objects. An Entry may be grouped with
other Entry objects with a Tag. A Tag is never shared with Entry instances
from more than one Container.
If I delete a Container instance, all of its Entry instances get deleted
via cascade. What I'd also like to happen is to cascade the delete to all
of the Tags as well. Most of the documentation I've seen for cascading
deletes talks about deleting parents cascading to their children. In this
case, it seems like I want to delete the parent (Tag) if I'm the last child
(Entry). I'd like to avoid putting the Container pk on the Tag since it's
already on the Entry and that seems like it's not properly normalized.
My example seems close to the "Many-to-many orphan deletion" given in
sqlalchemy-utils, except it's many-to-one.
http://sqlalchemy-utils.readthedocs.org/en/latest/listeners.html#many-to-many-orphan-deletion
Any ideas?
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