"dependency rule tried to blank out primary key" means:
1. A references B, B has a foreign key to A.
2. A is deleted.
3. each B associated with A must therefore have the foreign key of A set to
NULL (this is the default behavior if 'delete' cascade isn't configured).
4. the foreign key on B is also part of the primary key. SQLA detects this and
raises an error.
The solution, B must also be deleted when A is deleted.
In your example, you're probably trying to do this as I see cascade="all,
delete-orphan". Except this cascade must go on the side that points A to B, in
this case ObjectA.associations and ObjectB.associations (put it another way -
the side that does *not* have the foreign key on it). Since "associations"
is on the "backref" side of the declaration here, use the backref() function
and put "cascade='all, delete-orphan'" on that side.
On Aug 31, 2011, at 7:04 AM, Benjamin Sims wrote:
> Hi,
>
> I'm still having troubles correctly figuring my various many-to-many
> relations. Originally, this was a 'standard' many-to-many; that is, a
> secondary table was specified.
>
> However, I now need to add further information to the relation and am
> converting into an AssociationObject. My problem is in ensuring that the
> original calls still work. So, currently I have:
>
> class Association(Base):
>
> __tablename__ = 'objecta_objectb'
> objecta_id = Column(Integer, ForeignKey('objecta.id'), primary_key=True)
> objectb_id = Column(Integer, ForeignKey('objectb.id'), primary_key=True)
> objecta = relationship("ObjectA", backref = "associations", cascade =
> 'all, delete-orphan')
> objectb = relationship("ObjectB", backref = "associations", cascade =
> 'all, delete-orphan')
>
> def __init__(self, objecta = None, objectb = None):
> self.objecta = objecta
> self.objectb = objectb
> pass
>
> class ObjectA(Base):
> objectbs = association_proxy('associations', 'objectb', creator=lambda b:
> Association(objectb=b))
>
> class ObjectB(Base):
> objectas = association_proxy('associations', 'objecta', creator=lambda a:
> Association(objecta=a))
>
> The point of all this is so that I can create the associations by doing:
>
> objecta_instance.objectbs.append(objectb_instance)
>
> or
> association_instance = Association(objectb = objectb_instance)
> objecta_instance.associations.append(association_instance)
>
> Sorry if the above is rather convoluted; it is based on
> http://www.preetk.com/node/sqlalchemy-part-2-declarative-bi-directional-association-classes/,
> which is somewhat clearer.
>
> Anyway, using the above I get:
>
> AssertionError: Dependency rule tried to blank-out primary key column
> 'objecta_objectb.objectb_id' on instance '<Association at 0x109277b90>'
>
> What am I doing wrong? I understand that it is somehow related to how the
> cascade is set up, but I'm not sure how to move forward.
>
> Thanks,
> Ben
>
>
>
>
>
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