[Sqlalchemy-tickets] Issue #4215: AttributeError for `Mutable.associate_with` when using a relationship to non primary mapper (zzzeek/sqlalchemy)

[David M. <iss...@bi...>]

New issue 4215: AttributeError for `Mutable.associate_with` when using a relationship to non primary mapper
https://bitbucket.org/zzzeek/sqlalchemy/issues/4215/attributeerror-for-mutableassociate_with

David McDonald:

### Outline of the problem ###
I have several models that have JSON fields. I also have a relationship to non primary mapper as following the SQLAlchemy docs.

I tried to introduce mutability tracking to my JSON fields using https://github.com/edelooff/sqlalchemy-json. I ran into a problem with using `associate_with`. 

I've stripped back my code to remove the use of `sqlalchemy-json` and the problem still remains just when using `sqlalchemy.ext.mutable.Mutable`. `listen_for_type` seems to assume that all `mapper.column_attrs` are attributes existing on the original class.

I believe this is a bug but feel free to point out if I am doing something incorrect.

### Requirements ###
```
Python 3.6.4
Postgres 9.5.3
SQLAlchemy 1.2.5
psycopg2 2.7.4
```

### Code example ###
A full working example is available on Github for easy running - https://github.com/idavidmcdonald/sqlalchemy-bug-example

```
from sqlalchemy import Column, Integer, ForeignKey, join, create_engine
from sqlalchemy.dialects.postgresql import JSON
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.mutable import Mutable
from sqlalchemy.orm import mapper, relationship


# The following example is taken from the docs regarding a non primary relationship mapper
# http://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#relationship-to-non-primary-mapper
Base = declarative_base()

class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    b_id = Column(ForeignKey('b.id'))
    a_json = Column(JSON)

class B(Base):
    __tablename__ = 'b'

    id = Column(Integer, primary_key=True)
    b_json = Column(JSON)

class C(Base):
    __tablename__ = 'c'

    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey('a.id'))
    c_json = Column(JSON)

class D(Base):
    __tablename__ = 'd'

    id = Column(Integer, primary_key=True)
    c_id = Column(ForeignKey('c.id'))
    b_id = Column(ForeignKey('b.id'))
    d_json = Column(JSON)

# 1. set up the join() as a variable, so we can refer
# to it in the mapping multiple times.
j = join(B, D, D.b_id == B.id).join(C, C.id == D.c_id)

# 2. Create a new mapper() to B, with non_primary=True.
# Columns in the join with the same name must be
# disambiguated within the mapping, using named properties.
B_viacd = mapper(B, j, non_primary=True, properties={
    "b_id": [j.c.b_id, j.c.d_b_id],
    "d_id": j.c.d_id
    })

A.b = relationship(B_viacd, primaryjoin=A.b_id == B_viacd.c.b_id)

# Create DB
engine = create_engine('postgresql://localhost/test_db')
Base.metadata.create_all(engine)

# If we associate `Mutable` with `JSON` then `A` can not be instantiated succesfully
Mutable.associate_with(JSON)
item1 = A()
```

### Stack trace ###
```
Traceback (most recent call last):
  File "example.py", line 62, in <module>
    item1 = A()
  File "<string>", line 2, in __init__
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/orm/instrumentation.py", line 391, in _new_state_if_none
    state = self._state_constructor(instance, self)
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 767, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/orm/instrumentation.py", line 221, in _state_constructor
    self.dispatch.first_init(self, self.class_)
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/event/attr.py", line 284, in __call__
    fn(*args, **kw)
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 3139, in _event_on_first_init
    configure_mappers()
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 3032, in configure_mappers
    mapper, mapper.class_)
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/event/attr.py", line 246, in __call__
    fn(*args, **kw)
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/orm/events.py", line 621, in wrap
    fn(*arg, **kw)
  File "/Users/davidmcdonald/sqlalchemy-bug-example/venv/lib/python3.6/site-packages/sqlalchemy/ext/mutable.py", line 571, in listen_for_type
    cls.associate_with_attribute(getattr(class_, prop.key))
AttributeError: type object 'B' has no attribute 'd_json'
```