Multi-db test fails to run because of fixture containing M2M field (dumped with --natural-foreign)

[Edwin]

I noticed this issue in Django 1.5.10 but I've managed to replicate it in Django 1.7.1 as well.

I have a multi-db configuration but when I run a test case that uses a fixture containing many-to-many field (dumped using --natural-foreign option), it throws an error because it's trying to query a table in the secondary DB that only exists in the default DB when installing the fixture.

Looking at this code, it looks like Django tries install the fixture for all configured DB:

Line 899 (django/test/testcases.py):

for db_name in self._databases_names(include_mirrors=False):
    if self.fixtures:                                       
        try:                                                
            call_command('loaddata', *self.fixtures,        
                         **{                                
                             'verbosity': 0,                
                             'commit': False,               
                             'database': db_name,           
                             'skip_checks': True,           
                         })                                 
        except Exception:                                   
            self._fixture_teardown()                        
            raise                                           

...which is fine, but then here during serialization, it's making a query using the requested db without checking with "allow_migrate" first:

Line 112: django/core/serializers/python.py

# Handle M2M relations                                                                           
if field.rel and isinstance(field.rel, models.ManyToManyRel):                                    
    if hasattr(field.rel.to._default_manager, 'get_by_natural_key'):                             
        def m2m_convert(value):                                                                  
            if hasattr(value, '__iter__') and not isinstance(value, six.text_type):              
                return field.rel.to._default_manager.db_manager(db).get_by_natural_key(*value).pk
            else:                                                                                
                return smart_text(field.rel.to._meta.pk.to_python(value))                        
    else:                                                                                        
        m2m_convert = lambda v: smart_text(field.rel.to._meta.pk.to_python(v))                   
    m2m_data[field.name] = [m2m_convert(pk) for pk in field_value]                               

The line field.rel.to._default_manager.db_manager(db).get_by_natural_key(*value).pk doesn't check if the related model actually exists (or allowed) in db, causing a DeserializationError when fixture is being installed during unit test. In my example, I have a model that has a Many-to-Many reference to "Permission" table. I serialize this model with "dumpdata" command using "--natural-foreign" option.
However, when I run the test, the error I got was:

(django1.7)$ ./manage.py test
Creating test database for alias 'default'...
Creating test database for alias 'misc'...
E
======================================================================
ERROR: test_something (book.tests.MyTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/test/testcases.py", line 182, in __call__
    self._pre_setup()
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/test/testcases.py", line 754, in _pre_setup
    self._fixture_setup()
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/test/testcases.py", line 907, in _fixture_setup
    'skip_checks': True,
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 115, in call_command
    return klass.execute(*args, **defaults)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/management/base.py", line 338, in execute
    output = self.handle(*args, **options)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/management/commands/loaddata.py", line 61, in handle
    self.loaddata(fixture_labels)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/management/commands/loaddata.py", line 91, in loaddata
    self.load_label(fixture_label)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/management/commands/loaddata.py", line 142, in load_label
    for obj in objects:
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/serializers/json.py", line 81, in Deserializer
    six.reraise(DeserializationError, DeserializationError(e), sys.exc_info()[2])
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/serializers/json.py", line 75, in Deserializer
    for obj in PythonDeserializer(objects, **options):
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/serializers/python.py", line 122, in Deserializer
    m2m_data[field.name] = [m2m_convert(pk) for pk in field_value]
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/core/serializers/python.py", line 117, in m2m_convert
    return field.rel.to._default_manager.db_manager(db).get_by_natural_key(*value).pk
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/contrib/auth/models.py", line 35, in get_by_natural_key
    model),
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/models/manager.py", line 92, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/models/query.py", line 351, in get
    num = len(clone)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/models/query.py", line 122, in __len__
    self._fetch_all()
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/models/query.py", line 966, in _fetch_all
    self._result_cache = list(self.iterator())
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/models/query.py", line 265, in iterator
    for row in compiler.results_iter():
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 700, in results_iter
    for rows in self.execute_sql(MULTI):
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 786, in execute_sql
    cursor.execute(sql, params)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/home/ejaury/venvs/django1.7/local/lib/python2.7/site-packages/django/db/backends/sqlite3/base.py", line 485, in execute
    return Database.Cursor.execute(self, query, params)
DeserializationError: Problem installing fixture '/scratch/django/1.7/mybook/book/fixtures/permset_test.json': no such table: auth_permission

Here's my setup (note that I have simplified this example from my actual code, but it's still reproducible with this simplified code):

router.py

class MyRouter(object):                           
    def db_for_read(self, model, **hints):        
        return None                               
                                                  
    def db_for_write(self, model, **hints):       
        return None                               
                                                  
    def allow_relation(self, obj1, obj2, **hints):
        return True                               
                                                  
    def allow_migrate(self, db, model):           
        return (db == 'default')                  

database settings

DATABASES = {                                             
    'default': {                                          
        'ENGINE': 'django.db.backends.sqlite3',           
        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),     
    },                                                    
    'misc': {                                             
        'ENGINE': 'django.db.backends.sqlite3',           
        'NAME': os.path.join(BASE_DIR, 'db_misc.sqlite3'),
    },                                                    
}                                                         
DATABASE_ROUTERS = ['router.MyRouter']                    

models.py

from django.db import models     
from django.contrib.auth.models import Permission                      
                                                                       
                                                                       
class PermSet(models.Model):                                           
    name = models.CharField(max_length=100)                            
    permissions = models.ManyToManyField(Permission)
                                                                       
                                                                       
class AdminProfile(models.Model):                                      
    name = models.CharField(max_length=100)                            
    perm_sets = models.ManyToManyField(PermSet)                        

tests.py

from django.test import TestCase           
from book.models import PermSet          
                                      
class MyTest(TestCase):                  
    fixtures = ['permset_test.json']     
    multi_db = True                      
                                         
    def test_something(self):            
        pass   

This is my fixture:

permset_test.json

[
{
    "fields": {
        "name": "test",
        "permissions": [
            [
                "change_logentry",
                "admin",
                "logentry"
            ],
            [
                "delete_logentry",
                "admin",
                "logentry"
            ]
        ]
    },
    "model": "book.permset",
    "pk": 1
}
]

...which I dumped using this command:

(django1.7)$ ./manage.py dumpdata book.PermSet --indent=4 --natural-foreign > permset_test.json