Add BlankField and mount enums using it v3 (#1096)

* Add BlankField and mount enums using it

* fix lint error from duplicate import

Co-authored-by: Jonathan Kim <jkimbo@gmail.com>

graphene_django/converter.py

@@ -1,10 +1,11 @@
 from collections import OrderedDict
-from functools import singledispatch
+from functools import singledispatch, partial, wraps
 
 from django.db import models
 from django.utils.encoding import force_str
 from django.utils.functional import Promise
 from django.utils.module_loading import import_string
+
 from graphene import (
     ID,
     UUID,
@@ -22,6 +23,7 @@ from graphene import (
     Time,
     Decimal,
 )
+from graphene.types.resolver import get_default_resolver
 from graphene.types.json import JSONString
 from graphene.utils.str_converters import to_camel_case
 from graphql import GraphQLError, assert_valid_name
@@ -33,6 +35,24 @@ from .settings import graphene_settings
 from .utils.str_converters import to_const
 
 
+class BlankValueField(Field):
+    def get_resolver(self, parent_resolver):
+        resolver = self.resolver or parent_resolver
+
+        # create custom resolver
+        def blank_field_wrapper(func):
+            @wraps(func)
+            def wrapped_resolver(*args, **kwargs):
+                return_value = func(*args, **kwargs)
+                if return_value == "":
+                    return None
+                return return_value
+
+            return wrapped_resolver
+
+        return blank_field_wrapper(resolver)
+
+
 def convert_choice_name(name):
     name = to_const(force_str(name))
     try:
@@ -71,7 +91,8 @@ def convert_choices_to_named_enum_with_descriptions(name, choices):
         def description(self):
             return str(named_choices_descriptions[self.name])
 
-    return Enum(name, list(named_choices), type=EnumWithDescriptionsType)
+    return_type = Enum(name, list(named_choices), type=EnumWithDescriptionsType)
+    return return_type
 
 
 def generate_enum_name(django_model_meta, field):
@@ -108,11 +129,12 @@ def convert_django_field_with_choices(
             return converted
     choices = getattr(field, "choices", None)
     if choices and convert_choices_to_enum:
-        enum = convert_choice_field_to_enum(field)
+        EnumCls = convert_choice_field_to_enum(field)
         required = not (field.blank or field.null)
-        converted = enum(
+
+        converted = EnumCls(
             description=get_django_field_description(field), required=required
-        )
+        ).mount_as(BlankValueField)
     else:
         converted = convert_django_field(field, registry)
     if registry is not None:

graphene_django/tests/test_converter.py

@@ -164,8 +164,7 @@ def test_field_with_choices_convert_enum():
         class Meta:
             app_label = "test"
 
-    graphene_type = convert_django_field_with_choices(field)
-    assert isinstance(graphene_type, graphene.Enum)
+    graphene_type = convert_django_field_with_choices(field).type.of_type
     assert graphene_type._meta.name == "TestTranslatedModelLanguageChoices"
     assert graphene_type._meta.enum.__members__["ES"].value == "es"
     assert graphene_type._meta.enum.__members__["ES"].description == "Spanish"
@@ -418,3 +417,43 @@ def test_generate_v2_enum_name(graphene_settings):
         app_label="some_long_app_name", object_name="SomeObject"
     )
     assert generate_enum_name(model_meta, field) == "SomeObjectFizzBuzz"
+
+
+def test_choice_enum_blank_value():
+    """Test that choice fields with blank values work"""
+
+    class ReporterType(DjangoObjectType):
+        class Meta:
+            model = Reporter
+            fields = (
+                "first_name",
+                "a_choice",
+            )
+
+    class Query(graphene.ObjectType):
+        reporter = graphene.Field(ReporterType)
+
+        def resolve_reporter(root, info):
+            return Reporter.objects.first()
+
+    schema = graphene.Schema(query=Query)
+
+    # Create model with empty choice option
+    Reporter.objects.create(
+        first_name="Bridget", last_name="Jones", email="bridget@example.com"
+    )
+
+    result = schema.execute(
+        """
+        query {
+            reporter {
+                firstName
+                aChoice
+            }
+        }
+    """
+    )
+    assert not result.errors
+    assert result.data == {
+        "reporter": {"firstName": "Bridget", "aChoice": None},
+    }