Allow nullable foreign keys

djantic/main.py

@@ -1,4 +1,5 @@
 import inspect
+import sys
 from enum import Enum
 from functools import reduce
 from itertools import chain
@@ -15,6 +16,10 @@
 from pydantic.errors import PydanticUserError
 from pydantic._internal._model_construction import ModelMetaclass
 
+if sys.version_info >= (3, 10):
+    from types import UnionType
+else:
+    from typing import Union as UnionType
 
 from .fields import ModelSchemaField
 
@@ -136,6 +141,16 @@ def __new__(mcs, name: str, bases: tuple, namespace: dict, **kwargs):
         return cls
 
 
+def _is_optional_field(annotation) -> bool:
+    args = get_args(annotation)
+    return (
+            (get_origin(annotation) is Union or get_origin(annotation) is UnionType)
+            and type(None) in args
+            and len(args) == 2
+            and any(inspect.isclass(arg) and issubclass(arg, ModelSchema) for arg in args)
+    )
+
+
 class ProxyGetterNestedObj:
     def __init__(self, obj: Any, schema_class):
         self._obj = obj
@@ -199,7 +214,17 @@ def dict(self) -> dict:
                 # Pick the underlying annotation
                 annotation = get_args(annotation)[0]
 
-            if inspect.isclass(annotation) and issubclass(annotation, ModelSchema):
+            if _is_optional_field(annotation):
+                value = self.get(key)
+                if value is None:
+                    data[key] = None
+                else:
+                    non_none_type_annotation = next(
+                        arg for arg in get_args(annotation) if arg is not type(None)
+                    )
+                    data[key] = self._get_annotation_objects(value, non_none_type_annotation)
+
+            elif inspect.isclass(annotation) and issubclass(annotation, ModelSchema):
                 data[key] = self._get_annotation_objects(self.get(key), annotation)
             else:
                 key = fieldinfo.alias if fieldinfo.alias else key

tests/test_fields.py

@@ -1,6 +1,8 @@
+from typing import Optional
+
 import pytest
 from pydantic import ConfigDict
-from testapp.models import Configuration, Listing, Preference, Record, Searchable, User
+from testapp.models import Configuration, Listing, Preference, Record, Searchable, User, NullableChar, NullableFK
 
 from pydantic import (
     ValidationInfo,
@@ -408,3 +410,27 @@ class ListingSchema(ModelSchema):
         "id": None,
         "items": ["a", "b"],
     }
+
+
+@pytest.mark.django_db
+def test_nullable_fk():
+    class NullableCharSchema(ModelSchema):
+        model_config = ConfigDict(model=NullableChar, include='value')
+
+    class NullableFKSchema(ModelSchema):
+        nullable_char: Optional[NullableCharSchema] = None
+        model_config = ConfigDict(model=NullableFK, include='nullable_char')
+
+    nullable_char = NullableChar(value="test")
+    nullable_char.save()
+    model = NullableFK(nullable_char=nullable_char)
+    assert NullableFKSchema.from_django(model).dict() == {
+        "nullable_char": {
+            "value": "test"
+        }
+    }
+
+    model2 = NullableFK(nullable_char=None)
+    assert NullableFKSchema.from_django(model2).dict() == {
+        "nullable_char": None
+    }

tests/testapp/models.py

@@ -282,3 +282,11 @@ class Case(ExtendedModel):
 class Listing(models.Model):
     items = ArrayField(models.TextField(), size=4)
     content_type = models.ForeignKey(ContentType, on_delete=models.PROTECT, blank=True, null=True)
+
+
+class NullableChar(models.Model):
+    value = models.CharField(max_length=256, null=True, blank=True)
+
+
+class NullableFK(models.Model):
+    nullable_char = models.ForeignKey(NullableChar, null=True, blank=True, on_delete=models.CASCADE)