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).

model_bakery/baker.py

@@ -349,8 +349,21 @@ def _make(
                     self.m2m_dict[field.name] = self.m2m_value(field)
                 else:
                     self.m2m_dict[field.name] = self.model_attrs.pop(field.name)
+
             elif field.name not in self.model_attrs:
                 if (
+                    isinstance(field, ForeignKey)
+                    and "{0}_id".format(field.name) not in self.model_attrs
+                ):
+                    value = self.generate_value(field, commit_related)
+                    if isinstance(value, field.related_model):
+                        field_name = field.name
+                    else:
+                        field_name = "{0}_id".format(field.name)
+
+                    self.model_attrs[field_name] = value
+
+                elif (
                     not isinstance(field, ForeignKey)
                     or "{0}_id".format(field.name) not in self.model_attrs
                 ):

tests/generic/models.py

@@ -254,6 +254,23 @@ class DummyGenericRelationModel(models.Model):
     relation = GenericRelation(DummyGenericForeignKeyModel)
 
 
+class NamedThing(models.Model):
+    name = models.CharField(max_length=64)
+
+
+def get_default_namedthing_id():
+    instance, _ = NamedThing.objects.get_or_create(name="Default")
+    return instance.id
+
+
+class DummyForeignKeyWithDefaultIdModel(models.Model):
+    named_thing = models.ForeignKey(
+        "NamedThing",
+        default=get_default_namedthing_id,
+        on_delete=models.SET_DEFAULT,
+    )
+
+
 class DummyNullFieldsModel(models.Model):
     null_foreign_key = models.ForeignKey(
         "DummyBlankFieldsModel", null=True, on_delete=models.CASCADE

tests/test_filling_fields.py

@@ -276,6 +276,15 @@ 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.DummyForeignKeyWithDefaultIdModel, named_thing__name="Default"
+        )
+        assert dummy.named_thing.id == models.get_default_namedthing_id()
+
+
 @pytest.mark.django_db
 class TestsFillingFileField:
     def test_filling_file_field(self):