SearchFilter to support Custom query Transforms

Since Some fields support `__` as a custom Transform for query lookups we needed to update the m2m checking code to handle search_fields that contain __ that are not relationships.

rest_framework/filters.py

@@ -97,6 +97,9 @@ class SearchFilter(BaseFilterBackend):
                     if any(path.m2m for path in path_info):
                         # This field is a m2m relation so we know we need to call distinct
                         return True
+                else:
+                    # This field has a custom __ query transform but is not a relational field.
+                    break
         return False
 
     def filter_queryset(self, request, queryset, view):

tests/test_filters.py

@@ -1,9 +1,11 @@
 import datetime
 from importlib import reload as reload_module
 
+import django
 import pytest
 from django.core.exceptions import ImproperlyConfigured
 from django.db import models
+from django.db.models import CharField, Transform
 from django.db.models.functions import Concat, Upper
 from django.test import TestCase
 from django.test.utils import override_settings
@@ -189,6 +191,42 @@ class SearchFilterTests(TestCase):
 
         assert terms == ['asdf']
 
+    @pytest.mark.skipif(django.VERSION[:2] < (2, 2), reason="requires django 2.2 or higher")
+    def test_search_field_with_additional_transforms(self):
+        from django.test.utils import register_lookup
+
+        class SearchListView(generics.ListAPIView):
+            queryset = SearchFilterModel.objects.all()
+            serializer_class = SearchFilterSerializer
+            filter_backends = (filters.SearchFilter,)
+            search_fields = ('text__trim', )
+
+        view = SearchListView.as_view()
+
+        # an example custom transform, that trims `a` from the string.
+        class TrimA(Transform):
+            function = 'TRIM'
+            lookup_name = 'trim'
+
+            def as_sql(self, compiler, connection):
+                sql, params = compiler.compile(self.lhs)
+                return "trim(%s, 'a')" % sql, params
+
+        with register_lookup(CharField, TrimA):
+            # Search including `a`
+            request = factory.get('/', {'search': 'abc'})
+
+            response = view(request)
+            assert response.data == []
+
+            # Search excluding `a`
+            request = factory.get('/', {'search': 'bc'})
+            response = view(request)
+            assert response.data == [
+                {'id': 1, 'title': 'z', 'text': 'abc'},
+                {'id': 2, 'title': 'zz', 'text': 'bcd'},
+            ]
+
 
 class AttributeModel(models.Model):
     label = models.CharField(max_length=32)