Add back support for django-filter < 2

.travis.yml

@@ -45,6 +45,8 @@ matrix:
     env: TEST_TYPE=build DJANGO_VERSION=2.1
   - python: '3.6'
     env: TEST_TYPE=build DJANGO_VERSION=2.1
+  - python: '2.7'
+    env: TEST_TYPE=lint
   - python: '3.6'
     env: TEST_TYPE=lint
 deploy:

graphene_django/filter/filterset.py

@@ -1,7 +1,7 @@
 import itertools
 
 from django.db import models
-from django_filters import Filter, MultipleChoiceFilter
+from django_filters import Filter, MultipleChoiceFilter, VERSION
 from django_filters.filterset import BaseFilterSet, FilterSet
 from django_filters.filterset import FILTER_FOR_DBFIELD_DEFAULTS
 
@@ -51,6 +51,37 @@ class GrapheneFilterSetMixin(BaseFilterSet):
     )
 
 
+# To support a Django 1.11 + Python 2.7 combination django-filter must be
+# < 2.x.x. To support the earlier version of django-filter, the
+# filter_for_reverse_field method must be present on GrapheneFilterSetMixin and
+# must not be present for later versions of django-filter.
+if VERSION[0] < 2:
+    from django.utils.text import capfirst
+
+    class GrapheneFilterSetMixinPython2(GrapheneFilterSetMixin):
+
+        @classmethod
+        def filter_for_reverse_field(cls, f, name):
+            """Handles retrieving filters for reverse relationships
+             We override the default implementation so that we can handle
+            Global IDs (the default implementation expects database
+            primary keys)
+            """
+            try:
+                rel = f.field.remote_field
+            except AttributeError:
+                rel = f.field.rel
+            default = {"name": name, "label": capfirst(rel.related_name)}
+            if rel.multiple:
+                # For to-many relationships
+                return GlobalIDMultipleChoiceFilter(**default)
+            else:
+                # For to-one relationships
+                return GlobalIDFilter(**default)
+
+    GrapheneFilterSetMixin = GrapheneFilterSetMixinPython2
+
+
 def setup_filterset(filterset_class):
     """ Wrap a provided filterset in Graphene-specific functionality
     """