Resolve supplied `obj` to a Django model class.

    `obj` must be a Django model class itself, or a string
    representation of one.  Useful in situations like GH #1225 where
    Django may not have resolved a string-based reference to a model in
    another model's foreign key definition.

    String representations should have the format:
        'appname.ModelName'

def _resolve_model(obj):
    """
    Resolve supplied `obj` to a Django model class.

    `obj` must be a Django model class itself, or a string
    representation of one.  Useful in situations like GH #1225 where
    Django may not have resolved a string-based reference to a model in
    another model's foreign key definition.

    String representations should have the format:
        'appname.ModelName'
    """
    if isinstance(obj, six.string_types) and len(obj.split('.')) == 2:
        app_name, model_name = obj.split('.')
        resolved_model = apps.get_model(app_name, model_name)
        if resolved_model is None:
            msg = "Django did not return a model for {0}.{1}"
            raise ImproperlyConfigured(msg.format(app_name, model_name))
        return resolved_model
    elif inspect.isclass(obj) and issubclass(obj, models.Model):
        return obj
    raise ValueError("{0} is not a Django model".format(obj))