to_db_fields fix

[elephanter]

#1275 fix with tests

[wojcikstefan]

Thanks for this @elephanter! Does this PR mean that SomeDoc.objects.only('some_field_thats_not_on_the_doc') doesn't fail anymore?

[wojcikstefan]

Could re-write it without a double-negative, i.e. f if isinstance(f, six,string_types) else f.db_field

[wojcikstefan]

ditto

[elephanter]

Problem was in MapField, when trying to get value from it by key. (DictField will work correctly as I know)
_fields_to_dbfields method looking up fields with _lookup_field method, and expected result to be BaseField instance. But _lookup_field returns strings also. For example in case of Array field

            if field_name.isdigit() and isinstance(field, ListField):
                fields.append(field_name)
                continue

I think there was another error, where we requesting some index in array, like

obj.objects().only('arr.0.fld')
and it also was fixed. Maybe we need another test for that case

[wojcikstefan]

Right, I was just asking if a case that should fail doesn't anymore (which is my hunch). If that's the case, we can't merge this PR.

Just to reiterate, I believe SomeDoc.objects.only('some_field_thats_not_on_the_doc') fails on current master and it still should fail.

[elephanter]

ok. if that test should fail and do not fail now - travis tests must fail. Maybe I don't understand you quite well

[wojcikstefan]

Hi @elephanter! Sorry for the delay and the confusion. My hunch was wrong and, after a closer inspection of your PR, everything looks good. Thanks again!