Added support for multi_db

[pmourlanne]

This adds support a marker and a fixture, that make tests behave like Django TestCase's multi_db class attribute (as defined in #76).

I added empty tests to test the added fixture and marker. I'm not sure how to implement them. I'm willing to try if you help me with a few pointers :>

(I have added the same kind of multi_db support for pytest-django 2.9.1, I can do another PR if you are interested)

[blueyed]

I think a more universal approach would be to get access to the Django TestCase, where you then could set multi_db on. I will create a PR for that.
/cc @pelme

[blueyed]

Heh, since multi_db is a class attribute the following (in your conftest.py) is enough after all:

from django.test import TestCase
TestCase.multi_db = True

[blueyed]

FWIW, I am using the following now as a workaround until something like this and/or #431 gets in:

@pytest.fixture
def multi_db():
    pass

And in a plugin:

def pytest_generate_tests(metafunc):
    if 'multi_db' in metafunc.fixturenames:
        # Enable Django's multi_db feature for tests globally.
        # Can be improved after
        # https://github.com/pytest-dev/pytest-django/pull/431.
        if 'transactional_db' in metafunc.fixturenames:
            from djanto.test import TransactionTestCase
            TransactionTestCase.multi_db = True
        else:
            from django.test import TestCase
            TestCase.multi_db = True

(It could/should hook into / use django_db_modify_db_settings probably instead to be a bit cleaner)

[pmourlanne]

@blueyed I made the changes you requested (I think :o)

I still don't know how to write the tests for the new marker and fixture :/

[blueyed]

Don't worry. Let's wait for some feeback from others.
With e.g. #431 this might not be necessary after all.

Maybe also related (just closed it): #74

[pmourlanne]

Hello, I just updated this pull request :)

I had difficulties making the marker work. The solution used here, to check for the marker in _django_db_fixture_helper is not ideal, other solutions are welcome :)
Internal tests should be working with sqlite, I still need to work on other backends.

[bluetech]

I like the approach here, unfortunately multi_db is now deprecated, replaced by databases, so we should change to use that. I'll close this PR, as it's probably better to do that in a new PR, maybe basing on this one.

Thanks @pmourlanne!