OneToOne and ManyToMany models are nullable (#8)

These foreign key relationships should be nullable and as such this
should be reflected in the schema and when making a model instance into
a schema.

Prior to this this would throw an error, as it was not recognized as
nullable.

djantic/fields.py

@@ -2,7 +2,8 @@
 from datetime import date, datetime, time, timedelta
 from decimal import Decimal
 from enum import Enum
-from typing import Any, Dict, List, Union
+from typing import Any, Dict, List, Union, Optional
+import typing
 from uuid import UUID
 
 from django.utils.functional import Promise
@@ -130,9 +131,6 @@ def ModelSchemaField(field: Any, schema_name: str) -> tuple:
         if blank or null:
             python_type = Union[python_type, None]
 
-        if field.related_model:
-            field = field.target_field
-
     else:
         if field.choices:
             enum_choices = {}
@@ -182,21 +180,37 @@ def ModelSchemaField(field: Any, schema_name: str) -> tuple:
         title = field.verbose_name.title()
 
     if not description:
-        description = field.name
+        related_field_name = None
+        if field.related_model:
+            related_field_name = field.target_field.name
+
+        description = related_field_name or field.name
 
     if (
         getattr(field, "get_internal_type", lambda: None)() in STR_TYPES
         and not field.choices
     ):
         max_length = field.max_length
 
+    field_info = FieldInfo(
+        default=default,
+        default_factory=default_factory,
+        title=title,
+        description=str(description),
+        max_length=max_length,
+    )
+
+    field_is_optional = all([
+        getattr(field, "null", None),
+        field.is_relation,
+        # A list that is null, is the empty list. So there is no need
+        # to make it nullable.
+        typing.get_origin(python_type) is not list
+    ])
+    if field_is_optional:
+        python_type = Optional[python_type]
+
     return (
         python_type,
-        FieldInfo(
-            default=default,
-            default_factory=default_factory,
-            title=title,
-            description=str(description),
-            max_length=max_length,
-        ),
+        field_info
     )

tests/test_fields.py

@@ -1,10 +1,38 @@
 import pytest
 from pydantic import ConfigDict
-from testapp.models import Configuration, Listing, Preference, Record, Searchable
+from testapp.models import Configuration, Listing, Preference, Record, Searchable, User
 
 from djantic import ModelSchema
 
 
+@pytest.mark.django_db
+def test_schema_without_include_and_exclude():
+    """
+    Using a schema without include and exclude populates userschema correctly
+    Optional foreign key handled gracefully.
+    """
+
+    user = User.objects.create(
+        first_name="Jordan", last_name="Eremieff", email="[email protected]"
+    )
+
+    class UserSchema(ModelSchema):
+        model_config = ConfigDict(model=User)
+
+    dumped = UserSchema.from_django(user).model_dump()
+    # These values are here, but a hassle to use freeze gun.
+    dumped.pop("updated_at")
+    dumped.pop("created_at")
+
+    assert dumped == {
+        "first_name": "Jordan",
+        "id": 1,
+        "email": "[email protected]",
+        "profile": None,
+        "last_name": "Eremieff",
+    }
+
+
 @pytest.mark.django_db
 def test_unhandled_field_type():
     class SearchableSchema(ModelSchema):

tests/test_relations.py

@@ -364,10 +364,17 @@ class ProfileWithUserSchema(ModelSchema):
                 "description": "A user of the application.",
                 "properties": {
                     "profile": {
+                        "anyOf": [
+                            {
+                                "type": "integer",
+                            },
+                            {
+                                "type": "null",
+                            },
+                        ],
                         "default": None,
                         "description": "id",
                         "title": "Profile",
-                        "type": "integer",
                     },
                     "id": {
                         "anyOf": [{"type": "integer"}, {"type": "null"}],