Background: I have a two model classes where one is a parent of the other via a one to many relationship. The parent has a collection of the children with a backref:
class Parent(base): __tablename__ = "parent_table" parent_key = Column(String, primary_key=True) some_quantity = Column(Integer) children = relationship("Parent", backref="parent_table") class Child(base): __tablename__ = "child_table" child_key = Column(String, primary_key=True) some_quantity = Column(Integer) parent_key = Column(String, ForeignKey("parent_table.parent_key")) I'm running into the following problem: When I add a child to the parent collection, I have to make sure that the sum of the some_quantitiy fields of all children on the same parent do not exceed the some_quantity field of the parent. I tried adding a validator like so: -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at https://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.