ForeignKey Field validates using the default database rather than the current model instance's database

[pseidel@…]

When using multiple databases, ForeignKey fields can be setup to pull their options from a specific database.
ForeignKey.formfield({ 'using' : 'NotDefault' }).
However when validating the form, the ForeignKey fields do not validate using the associated database.

Current form validation method incorrectly validates ForeignKey relationships using the default manager instead of the model instances' database.

Django 1.2 Code: django.db.models.fields.related.py

class ForeignKey(RelatedField, Field):

    def validate(self, value, model_instance):
        if self.rel.parent_link:
            return
        super(ForeignKey, self).validate(value, model_instance)
        if value is None:
            return

        qs = self.rel.to._default_manager.filter(**{self.rel.field_name:value})
        qs = qs.complex_filter(self.rel.limit_choices_to)
        if not qs.exists():
            raise exceptions.ValidationError(self.error_messages['invalid'] % {
                'model': self.rel.to._meta.verbose_name, 'pk': value})

Proposed change will using the model_instances' database instance of the default manager.
As you can see self.rel.to._default_manager becomes self.rel.to._default_manager.using(model_instance._state.db).

    def validate(self, value, model_instance):
        if self.rel.parent_link:
            return
        super(ForeignKey, self).validate(value, model_instance)
        if value is None:
            return

        qs = self.rel.to._default_manager.using(model_instance._state.db).filter(**{self.rel.field_name:value})
        qs = qs.complex_filter(self.rel.limit_choices_to)
        if not qs.exists():
            raise exceptions.ValidationError(self.error_messages['invalid'] % {
                'model': self.rel.to._meta.verbose_name, 'pk': value})

[Ramiro Morales]

Duplicate of #14691 (That is newer but got picked and fixed first). Thanks for the report and fix suggestion.