Equality operator on django.db.models.Model not commutative
|Reported by:||Owned by:||anonymous|
|Component:||Database layer (models, ORM)||Version:||1.3|
|Cc:||anssi.kaariainen@…||Triage Stage:||Ready for checkin|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
Description (last modified by )
In math is is a common assumption that the equality operator should be commutative, i.e. A==B would always correspond to B==A.
In django's Model class I found non-commutative behaviour.
class Model(object): ... def __eq__(self, other): return isinstance(other, self.__class__) and self._get_pk_val() == other._get_pk_val() ...
This implementation of eq() will cause a different outcome between (A==B) and (B==A) in cases where one of the two compared objects is a derived class of the other.
class MyModel(models.Model): pass class MyDerivedModel(MyModel): pass A = MyModel() B = MyDerivedModel() (A==B) # --> A.__eq__ returns True if (a.pk == b.pk) (B==A) # --> B.__eq__ returns False
I checked how derived models are created in the database (MySQL 5.0 backend). The table for MyDerivedModel does not contain its own primary key, but a reference to the PK in MyModel instead. This means that there can never be a collision between the primary key of MyModel and the primary key of MyDerivedModel.
So it is safe to compare PKs across base and derived classes, as is currently done.
Make the type comparison between self and other symmetrical.
class Model(object): def __eq__(self, other): return (isinstance(other, self.__class__) or isinstance(self, other.__class__)) and \ (self._get_pk_val() == other._get_pk_val())