[IMP] models: avoid searching by id in read_group

When using read_group() and grouping by many2one fields, the many2one groupby
fields get updated from an integer id to a tuple from name_get() at the end
of the method.

e.g.: {'user_id': 1, ...} -> {'user_id': (1, u'Administrator'), ...}

To do this, it reads the many2one field by browsing on self.

When self is a model with _auto = False (typically an SQL view), searching by
id can be costly as the id column may not have usual properties for performance
(in some cases a full computation of the table is required to look up 1 id).

odoo/models.py

@@ -2025,13 +2025,7 @@ class BaseModel(object):
         if not groupby_fields:
             return fetched_data
 
-        many2onefields = [gb['field'] for gb in annotated_groupbys if gb['type'] == 'many2one']
-        if many2onefields:
-            data_ids = [r['id'] for r in fetched_data]
-            many2onefields = list(set(many2onefields))
-            data_dict = {d['id']: d for d in self.browse(data_ids).read(many2onefields)}
-            for d in fetched_data:
-                d.update(data_dict[d['id']])
+        self._read_group_resolve_many2one_fields(fetched_data, annotated_groupbys)
 
         data = map(lambda r: {k: self._read_group_prepare_data(k,v, groupby_dict) for k,v in r.iteritems()}, fetched_data)
         result = [self._read_group_format_result(d, annotated_groupbys, groupby, domain) for d in data]
@@ -2046,6 +2040,15 @@ class BaseModel(object):
             )
         return result
 
+    def _read_group_resolve_many2one_fields(self, data, fields):
+        many2onefields = {field['field'] for field in fields if field['type'] == 'many2one'}
+        for field in many2onefields:
+            ids_set = {d[field] for d in data}
+            m2o_records = self.env[self._fields[field].comodel_name].browse(ids_set)
+            data_dict = dict(m2o_records.name_get())
+            for d in data:
+                d[field] = (d[field], data_dict[d[field]]) if d[field] else False
+
     def _inherits_join_add(self, current_model, parent_model_name, query):
         """
         Add missing table SELECT and JOIN clause to ``query`` for reaching the parent table (no duplicates)