meta_fields implementation assumes that Meta.fields is explicitly defined

[leifurhauks]

The implementation of the meta_fields functionality breaks if a serializer doesn't have an explicitly defined fields attribute on its Meta. That attribute is not required by DRF; if omitted on a ModelSerializer all the model fields are used, minus any fields listed in Meta.exclude. While it is arguably better practice to explicitly define Meta.fields, the implicit form should be supported for compatibility with upstream.

The crash occurs in the __init__ of rest_framework_json_api.serializers.ModelSerializer: a763ace#diff-8f2fdb5cb1d819dce410d6b74d0b77b6R143

I'm working on a possible fix.

[leifurhauks]

I realized that creating a ModelSerializer without either the fields or exclude attributes is being deprecated upstream. This issue still needs to be fixed because the current implementation still breaks with serializers that use exclude instead of fields, and serializers that use the fields = '__all__' shortcut.

[jerel]

Would it make sense to just throw an exception if fields is missing?

[leifurhauks]

That would make it impossible to use exclude or fields = '__all__'.

I think it might be necessary to override the get_fields method on ModelSerializer. That would at least make it possible to use fields = '_all__'. I'm not sure about the use of exclude.