  from django.db.models.options import Options
  from django.db.models.query import Q
  from django.db.models.utils import make_model_tuple
  from django.utils import six
  from django.utils.encoding import (
      force_str, force_text, python_2_unicode_compatible,
  )
  from django.utils.functional import curry
  from django.utils.six.moves import zip
  from django.utils.text import capfirst, get_text_list
 
</s> remove from django.db.models.manager import ensure_default_manager
 </s> add from django.db.models.manager import Manager </s> remove from django.test import TestCase
 </s> add from django.test import TestCase, ignore_warnings </s> remove from django.test import TestCase
 </s> add from django.test import TestCase, ignore_warnings
from django.utils.deprecation import RemovedInDjango20Warning
from django.utils.deprecation import RemovedInDjango20Warning
  from django.db.models.options import Options
  from django.db.models.query import Q
  from django.db.models.utils import make_model_tuple
  from django.utils import six
from django.utils.deprecation import RemovedInDjango20Warning
  from django.utils.encoding import (
      force_str, force_text, python_2_unicode_compatible,
  )
  from django.utils.functional import curry
  from django.utils.six.moves import zip
  from django.utils.text import capfirst, get_text_list
keep keep keep add keep keep keep keep keep keep
 Fixed #10506, #13793, #14891, #25201 -- Introduced new APIs to specify models' default and base managers.

This deprecates use_for_related_fields.

Old API:

class CustomManager(models.Model):
    use_for_related_fields = True

class Model(models.Model):
    custom_manager = CustomManager()

New API:

class Model(models.Model):
    custom_manager = CustomManager()

    class Meta:
        base_manager_name = 'custom_manager'

Refs #20932, #25897.

Thanks Carl Meyer for the guidance throughout this work.
Thanks Tim Graham for writing the docs. 
django/db/models/base.py