Use read to fetch reverse field value of x2M field in field get

This avoid to create instances for origin or target just to get their id.

Closes #13245

trytond/trytond/model/fields/many2many.py

@@ -139,6 +139,7 @@ class Many2Many(Field):
 
         Relation = self.get_relation()
         origin_field = Relation._fields[self.origin]
+        reference_key = origin_field._type == 'reference'
 
         if origin_field.sortable(Relation):
             if origin_field._type == 'reference':
@@ -154,7 +155,7 @@ class Many2Many(Field):
 
         relations = []
         for sub_ids in grouped_slice(ids):
-            if origin_field._type == 'reference':
+            if reference_key:
                 references = ['%s,%s' % (model.__name__, x) for x in sub_ids]
                 clause = [(self.origin, 'in', references)]
             else:
@@ -162,12 +163,18 @@ class Many2Many(Field):
             clause += [(self.target, '!=', None)]
             if self.filter:
                 clause.append((self.target, 'where', self.filter))
-            relations.append(Relation.search(clause, order=order))
-        relations = Relation.browse(list(chain(*relations)))
+            relations.append(
+                [r.id for r in Relation.search(clause, order=order)])
+        relations = Relation.read(
+            list(chain(*relations)), [self.origin, self.target])
 
         for relation in relations:
-            origin_id = getattr(relation, self.origin).id
-            res[origin_id].append(getattr(relation, self.target).id)
+            if reference_key:
+                _, origin_id = relation[self.origin].split(',', 1)
+                origin_id = int(origin_id)
+            else:
+                origin_id = relation[self.origin]
+            res[origin_id].append(relation[self.target])
         return dict((key, tuple(value)) for key, value in res.items())
 
     def set(self, Model, name, ids, values, *args):

trytond/trytond/model/fields/one2many.py

@@ -142,12 +142,13 @@ class One2Many(Field):
         '''
         Target = self.get_target()
         field = Target._fields[self.field]
+        reference_key = field._type == 'reference'
         res = {}
         for i in ids:
             res[i] = []
 
         if field.sortable(Target):
-            if field._type == 'reference':
+            if reference_key:
                 order = [(self.field, None)]
             else:
                 order = [(self.field + '.id', None)]
@@ -159,19 +160,23 @@ class One2Many(Field):
             order += Target._order
         targets = []
         for sub_ids in grouped_slice(ids):
-            if field._type == 'reference':
+            if reference_key:
                 references = ['%s,%s' % (model.__name__, x) for x in sub_ids]
                 clause = [(self.field, 'in', references)]
             else:
                 clause = [(self.field, 'in', list(sub_ids))]
             if self.filter:
                 clause.append(self.filter)
-            targets.append(Target.search(clause, order=order))
-        targets = Target.browse(list(chain(*targets)))
+            targets.append([r.id for r in Target.search(clause, order=order)])
+        targets = Target.read(list(chain(*targets)), ['id', self.field])
 
         for target in targets:
-            origin_id = getattr(target, self.field).id
-            res[origin_id].append(target.id)
+            if reference_key:
+                _, origin_id = target[self.field].split(',', 1)
+                origin_id = int(origin_id)
+            else:
+                origin_id = target[self.field]
+            res[origin_id].append(target['id'])
         return dict((key, tuple(value)) for key, value in res.items())
 
     def set(self, Model, name, ids, values, *args):