Quick-and-dirty fix for forms using ModelChoiceField with to_field_name set.

form_fixes.py

# Quick-and-dirty fix for forms using ModelChoiceField with to_field_name set.
# See https://code.djangoproject.com/ticket/20202 for details.

# Example usage:
# >>> import form_fixes
# >>> form_fixes.fix_to_field_name()

from django import forms
from django.db import models

def fix_to_field_name():
    class TestModel(models.Model):
        key = models.CharField(max_length=32, unique=True)
        value = models.ForeignKey("TestModel", blank=True, null=True)

        class Meta:
            managed = False
            app_label = "test"

    class FakeQueryset(object):
        def all(self):
            m1 = TestModel(id=1, key="A", value=None)
            m2 = TestModel(id=2, key="B", value=m1)
            return [m1, m2]

    class TestForm(forms.ModelForm):
        value = forms.ModelChoiceField(FakeQueryset(), to_field_name="key")

        class Meta:
            model = TestModel
            fields = ("value",)

    m1, m2 = FakeQueryset().all()
    f = TestForm(instance=m2)
    if f.initial["value"] not in (m1, m1.key):
        original_init = forms.BaseModelForm.__init__

        def __init__(self, *args, **kwargs):
            if "instance" in kwargs:
                if not "initial" in kwargs:
                    kwargs["initial"] = {}
                # Workaround for Django 1.4 bug, as model_to_dict uses PK, and
                # does not know anything about form field's to_field_name.
                instance = kwargs["instance"]
                for name, field in self.declared_fields.items():
                    to_name = getattr(field, "to_field_name", None)
                    if to_name is not None:
                        f = getattr(instance, name)
                        if f is not None:
                            # ManyRelatedManager is a dynamically-generated class, thus the weird-looking check.
                            if f.__class__.__name__ == "ManyRelatedManager" and hasattr(f, "values_list"):
                                kwargs["initial"][name] = f.values_list(to_name, flat=True)
                            else:
                                kwargs["initial"][name] = getattr(f, to_name)
            return original_init(self, *args, **kwargs)

        forms.BaseModelForm.__init__ = __init__
        return True

    return False

if __name__ == "__main__":
    patched = fix_to_field_name()
    if patched:
        assert not fix_to_field_name(), "Patch failed!"
        print "Successfully patched"
    else:
        print "OK"