id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
22555	Migration doesn't work when fields are added using contribute_to_class	Matt3o12	nobody	"When you create a custom Field that implements the `contribute_to_class` and add another Field that way, it won't be possible to use migration. 

The code I used:

{{{
#!python
class MyField(models.CharField):
    def contribute_to_class(self, cls, name):
        if not cls._meta.abstract:
            field = CharField(max_length = 20)
            cls.add_to_class(""%s_mycontribute"" % name, field)
        
        
        super(MyField, self).contribute_to_class(cls, name)

}}}

What happens if the original model is used (`app.models.MyModel`):
1. contribute_to_class is called and adds additional fields (in my example, `MyField`). 
2. contribute_to_class calls it super function and adds the original field (`CharField`).  

 Everything works. The model has 2 fields: `MyField`, and `CharField`.

---

When python tries to create a migration (now the model `__fake__.MyModel` is used).
1. contribute_to_class is called. The model already has the contributed CharField (with the name %s_mycontribute) (why does the model already have it?).
2. MyField with the exactly name is added again (and no exception is thrown?!).
3. The original field (MyField) is added.

 The view now has 3 fields: `MyField`, `CharField`, and `CharField` again (both `CharField`s have the same name).
The next step django does is trying to migrate my model but it fails with the exception: ""django.db.utils.OperationalError: duplicate column name: charFile_mycontribute"" because the field was created twice.


My guess:
The original model is somewhere copied and the fields in _meta are not cleaned correctly. 
"	Bug	closed	Migrations	1.7-beta-2	Release blocker	invalid	migration, fake, __fake__, contribute_to_class		Unreviewed	0	0	0	0	0	0