Fix creation of model instances with related model fields (#164)

* Allow id to be used as FK default (Fix #136)

#117 introduced an issue for cases where `id` values is provided as a default for foreign keys.
This PR attempts to fix that issue by checking if generated value for FK field is an instance of this model - if not it uses `_id` as a field name (https://docs.djangoproject.com/en/3.1/ref/models/fields/#database-representation).

* Another fix idea: use default unless given custom argument

* Remove old way, do some cleanup

* Update CHANGELOG.md

CHANGELOG.md

@@ -18,7 +18,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 - Type hinting fixed for Recipe "_model" parameter  [PR #124](https://github.com/model-bakers/model_bakery/pull/124)
-- Modify `setup.py` to not import the whole module for package data, but get it from `__about__.py`  [PR #142](https://github.com/model-bakers/model_bakery/pull/142)
+- Fixed a bug (introduced in 1.2.1) that was breaking creation of model instances with related model fields [PR #164](https://github.com/model-bakers/model_bakery/pull/164)
+- [dev] Modify `setup.py` to not import the whole module for package data, but get it from `__about__.py`  [PR #142](https://github.com/model-bakers/model_bakery/pull/142)
 - [dev] Add Dependabot config file [PR #146](https://github.com/model-bakers/model_bakery/pull/146)
 - [dev] Update Dependabot config file to support GH Actions and auto-rebase [PR #160](https://github.com/model-bakers/model_bakery/pull/160)
 

model_bakery/baker.py

@@ -531,10 +531,11 @@ def generate_value(self, field: Field, commit: bool = True) -> Any:
         """Call the associated generator with a field passing all required args.
 
         Generator Resolution Precedence Order:
-        -- attr_mapping - mapping per attribute name
-        -- choices -- mapping from avaiable field choices
-        -- type_mapping - mapping from user defined type associated generators
-        -- default_mapping - mapping from pre-defined type associated
+        -- `field.default` - model field default value, unless explicitly overwritten during baking
+        -- `attr_mapping` - mapping per attribute name
+        -- `choices` -- mapping from available field choices
+        -- `type_mapping` - mapping from user defined type associated generators
+        -- `default_mapping` - mapping from pre-defined type associated
            generators
 
         `attr_mapping` and `type_mapping` can be defined easily overwriting the
@@ -543,8 +544,8 @@ def generate_value(self, field: Field, commit: bool = True) -> Any:
         is_content_type_fk = isinstance(field, ForeignKey) and issubclass(
             self._remote_field(field).model, contenttypes.models.ContentType
         )
-
-        if field.has_default():
+        # we only use default unless the field is overwritten in `self.rel_fields`
+        if field.has_default() and field.name not in self.rel_fields:
             if callable(field.default):
                 return field.default()
             return field.default

tests/generic/models.py

@@ -181,6 +181,10 @@ class LonelyPerson(models.Model):
     only_friend = models.OneToOneField(Person, on_delete=models.CASCADE)
 
 
+class Cake(models.Model):
+    name = models.CharField(max_length=64)
+
+
 class RelatedNamesModel(models.Model):
     name = models.CharField(max_length=256)
     one_to_one = models.OneToOneField(
@@ -191,6 +195,30 @@ class RelatedNamesModel(models.Model):
     )
 
 
+def get_default_cake_id():
+    instance, _ = Cake.objects.get_or_create(name="Muffin")
+    return instance.id
+
+
+class RelatedNamesWithDefaultsModel(models.Model):
+    name = models.CharField(max_length=256, default="Bravo")
+    cake = models.ForeignKey(
+        Cake,
+        on_delete=models.CASCADE,
+        default=get_default_cake_id,
+    )
+
+
+class RelatedNamesWithEmptyDefaultsModel(models.Model):
+    name = models.CharField(max_length=256, default="Bravo")
+    cake = models.ForeignKey(
+        Cake,
+        on_delete=models.CASCADE,
+        null=True,
+        default=None,
+    )
+
+
 class ModelWithOverridedSave(Dog):
     def save(self, *args, **kwargs):
         self.owner = kwargs.pop("owner")

tests/test_filling_fields.py

@@ -276,6 +276,37 @@ def test_filling_content_type_field(self):
         assert dummy.content_type.model_class() is not None
 
 
+@pytest.mark.django_db
+class TestFillingForeignKeyFieldWithDefaultFunctionReturningId:
+    def test_filling_foreignkey_with_default_id(self):
+        dummy = baker.make(models.RelatedNamesWithDefaultsModel)
+        assert dummy.cake.__class__.objects.count() == 1
+        assert dummy.cake.id == models.get_default_cake_id()
+        assert dummy.cake.name == "Muffin"
+
+    def test_filling_foreignkey_with_default_id_with_custom_arguments(self):
+        dummy = baker.make(
+            models.RelatedNamesWithDefaultsModel, cake__name="Baumkuchen"
+        )
+        assert dummy.cake.__class__.objects.count() == 1
+        assert dummy.cake.id == dummy.cake.__class__.objects.get().id
+        assert dummy.cake.name == "Baumkuchen"
+
+
+@pytest.mark.django_db
+class TestFillingOptionalForeignKeyField:
+    def test_not_filling_optional_foreignkey(self):
+        dummy = baker.make(models.RelatedNamesWithEmptyDefaultsModel)
+        assert dummy.cake is None
+
+    def test_filling_optional_foreignkey_implicitly(self):
+        dummy = baker.make(
+            models.RelatedNamesWithEmptyDefaultsModel, cake__name="Carrot cake"
+        )
+        assert dummy.cake.__class__.objects.count() == 1
+        assert dummy.cake.name == "Carrot cake"
+
+
 @pytest.mark.django_db
 class TestsFillingFileField:
     def test_filling_file_field(self):