Handle ForeignKey.to_field when retrieving default FK instance

See: https://docs.djangoproject.com/en/3.1/ref/models/fields/#django.db.models.ForeignKey.to_field

model_bakery/baker.py

@@ -538,8 +538,14 @@ def generate_value(self, field: Field, commit: bool = True) -> Any:
             if callable(default):
                 default = default()
             if is_fk:
-                # Pass instance instead of pk value
-                default = remote_field.model.objects.get(pk=default)
+                # Pass instance instead of pk/<to_field> value. In case a c
+                # custom to_field is set, Django should populate 
+                # field.to_fields with a single string.
+                key = "pk"
+                if field.to_fields and field.to_fields[0] is not None:
+                    key = field.to_fields[0]
+                instance_kwargs = {key: default}
+                default = remote_field.model.objects.get(**instance_kwargs)
             return default
         elif field.name in self.attr_mapping:
             generator = self.attr_mapping[field.name]

tests/generic/models.py

@@ -255,18 +255,19 @@ class DummyGenericRelationModel(models.Model):
 
 
 class NamedThing(models.Model):
-    name = models.CharField(max_length=64)
+    name = models.CharField(primary_key=True, max_length=64)
 
 
-def get_default_namedthing_id():
+def get_default_namedthing_name():
     instance, _ = NamedThing.objects.get_or_create(name="Default")
-    return instance.id
+    return instance.name
 
 
 class DummyForeignKeyWithDefaultIdModel(models.Model):
     named_thing = models.ForeignKey(
-        "NamedThing",
-        default=get_default_namedthing_id,
+        NamedThing,
+        default=get_default_namedthing_name,
+        to_field="name",
         on_delete=models.SET_DEFAULT,
     )
 

tests/test_filling_fields.py

@@ -279,8 +279,11 @@ def test_filling_content_type_field(self):
 @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()
+        dummy = baker.make(
+            models.DummyForeignKeyWithDefaultIdModel, 
+            named_thing__name="Default",
+        )
+        assert dummy.named_thing.name == models.get_default_namedthing_name()
 
 
 @pytest.mark.django_db