Store target of field as relation

For Many2Many it replaces the relation model by the target which is what is
expected by usage in many places like the domain for email models.

Closes #13756

trytond/CHANGELOG

@@ -1,3 +1,4 @@
+* Store target of field as relation
 * Remove support for Python 3.8
 * Add support for Python 3.13
 * Add on_write to ModelStorage

trytond/trytond/ir/model.py

@@ -344,7 +344,6 @@ class ModelField(
 
     @classmethod
     def register(cls, model, module_name, model_id):
-        pool = Pool()
         cursor = Transaction().connection.cursor()
 
         ir_model_field = cls.__table__()
@@ -363,17 +362,13 @@ class ModelField(
         model_fields = {f['name']: f for f in cursor_dict(cursor)}
 
         for field_name, field in model._fields.items():
-            if hasattr(field, 'model_name'):
-                relation = field.model_name
-            elif hasattr(field, 'relation_name'):
-                relation = field.relation_name
+            if hasattr(field, 'get_target'):
+                Relation = field.get_target()
+                relation = Relation.__name__
+                Model.register(Relation, module_name)
             else:
                 relation = None
 
-            if relation:
-                Relation = pool.get(relation)
-                Model.register(Relation, module_name)
-
             access = field_name in model.__access__
 
             if field_name not in model_fields: