some refactors in relations module

ormar/models/modelproxy.py

@@ -1,13 +1,17 @@
 import inspect
-from typing import List, Optional, Set, TYPE_CHECKING
+from typing import List, Optional, Set, TYPE_CHECKING, Type, TypeVar
 
 import ormar
+from ormar.exceptions import RelationshipInstanceError
+from ormar.fields import BaseField
 from ormar.fields.foreign_key import ForeignKeyField
 from ormar.models.metaclass import ModelMeta
 
 if TYPE_CHECKING:  # pragma no cover
     from ormar import Model
 
+Field = TypeVar("Field", bound=BaseField)
+
 
 class ModelTableProxy:
     if TYPE_CHECKING:  # pragma no cover
@@ -89,6 +93,17 @@ class ModelTableProxy:
                 if field.to == related.__class__ or field.to.Meta == related.Meta:
                     return name
 
+    @staticmethod
+    def resolve_relation_field(item: "Model", related: "Model") -> Type[Field]:
+        name = ModelTableProxy.resolve_relation_name(item, related)
+        to_field = item.Meta.model_fields.get(name)
+        if not to_field:  # pragma no cover
+            raise RelationshipInstanceError(
+                f"Model {item.__class__} does not have "
+                f"reference to model {related.__class__}"
+            )
+        return to_field
+
     @classmethod
     def merge_instances_list(cls, result_rows: List["Model"]) -> List["Model"]:
         merged_rows = []

ormar/relations/relation.py

@@ -39,7 +39,7 @@ class Relation:
     def _find_existing(self, child: "Model") -> Optional[int]:
         for ind, relation_child in enumerate(self.related_models[:]):
             try:
-                if relation_child.__same__(child):
+                if relation_child == child:
                     return ind
             except ReferenceError:  # pragma no cover
                 self.related_models.pop(ind)

ormar/relations/relation_manager.py

@@ -2,7 +2,6 @@ from typing import List, Optional, TYPE_CHECKING, Tuple, Type, Union
 from weakref import proxy
 
 import ormar
-from ormar.exceptions import RelationshipInstanceError
 from ormar.fields.foreign_key import ForeignKeyField
 from ormar.fields.many_to_many import ManyToManyField
 from ormar.relations import Relation
@@ -85,20 +84,7 @@ class RelationsManager:
 
     @staticmethod
     def add(parent: "Model", child: "Model", child_name: str, virtual: bool) -> None:
-        to_field = next(
+        to_field = child.resolve_relation_field(child, parent)
-            (
-                field
-                for field in child._orm._related_fields
-                if field.to == parent.__class__ or field.to.Meta == parent.Meta
-            ),
-            None,
-        )
-
-        if not to_field:  # pragma no cover
-            raise RelationshipInstanceError(
-                f"Model {child.__class__} does not have "
-                f"reference to model {parent.__class__}"
-            )
 
         (
             parent,