[Sqlalchemy-tickets] Issue #3708: Inconsistent behavior when updating foreign key (zzzeek/sqlalchemy)

[pkyosx <iss...@bi...>]

New issue 3708: Inconsistent behavior when updating foreign key
https://bitbucket.org/zzzeek/sqlalchemy/issues/3708/inconsistent-behavior-when-updating

pkyosx:

Suppose we have Parent and Child classes as One-To-Many relationship. If I move child from one parent to another, the order of changing foreign_key and foreign_object result in different behavior. Following are the briefing of reproducing steps. Attachments are runnable reproducing scripts. It still happens in 1.1.0b1.dev0 build. 


```
#!python

// Models
class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", back_populates="parent")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent", back_populates="children")

// Initial Setting
p1 = Parent()
p2 = Parent()
c = Child()
c.parent = p1
s.commit()

// Scenario1: Move c from p1 to p2
c.parent_id = p2.id
c.parent = p2
assert c in p2.children, "this will fail"

// Scenario 2: Move c from p1 to p2
c.parent = p2
c.parent_id = p2.id
assert c in p2.children, "this will success"
```