Fix x-nullable on serializers with allow_null=True (#566)

src/drf_yasg/inspectors/base.py

@@ -281,7 +281,7 @@ def SwaggerType(existing_object=None, **instance_kwargs):
                 instance_kwargs.setdefault('title', title)
             if description is not None:
                 instance_kwargs.setdefault('description', description)
-            if field.allow_null and not instance_kwargs.get('required', False) and not field.required:
+            if field.allow_null:
                 instance_kwargs['x_nullable'] = True
 
             instance_kwargs.update(kwargs)

testproj/snippets/models.py

@@ -16,6 +16,10 @@ class Snippet(models.Model):
     class Meta:
         ordering = ('created',)
 
+    @property
+    def nullable_secondary_language(self):
+        return None
+
 
 class SnippetViewer(models.Model):
     snippet = models.ForeignKey(Snippet, on_delete=models.CASCADE, related_name='viewers')

testproj/snippets/serializers.py

@@ -79,6 +79,8 @@ class SnippetSerializer(serializers.Serializer):
     rate = serializers.DecimalField(max_digits=6, decimal_places=3, default=Decimal('0.0'), coerce_to_string=False,
                                     validators=[MinValueValidator(Decimal('0.0'))])
 
+    nullable_secondary_language = LanguageSerializer(allow_null=True)
+
     def create(self, validated_data):
         """
         Create and return a new `Snippet` instance, given the validated data.

tests/reference.yaml

@@ -1130,6 +1130,7 @@ definitions:
       - code
       - tags
       - language
+      - nullableSecondaryLanguage
     type: object
     properties:
       id:
@@ -1230,6 +1231,25 @@ definitions:
         format: decimal
         default: 0.0
         minimum: 0.0
+      nullableSecondaryLanguage:
+        type: object
+        properties:
+          name:
+            title: Name
+            description: The name of the programming language
+            type: string
+            enum:
+              - cpp
+              - js
+              - python
+            default: python
+          readOnlyNullable:
+            title: Read only nullable
+            type: string
+            readOnly: true
+            minLength: 1
+            x-nullable: true
+        x-nullable: true
   SnippetViewer:
     required:
       - snippet