[IMP] core: save one query when reading one2many on a single record

For reading one2many fields, we make 2 queries (in batch):
- One to search "id" the comodel
- One to read <inverse_field> in the comodel to group lines by record
(sometimes, this one is bypass because the cache already contains the information).

In many cases (> 80% of cases in our tests) we read a one2many field on
a single record.  In that case, we can avoid the second query completely
(grouping all lines on a single record is trivial).

This reduces SQL queries by about 0.8% on all-install runbot builds.

closes odoo/odoo#99415

Signed-off-by: Rémy Voet <ryv@odoo.com>

odoo/fields.py

@@ -4165,11 +4165,18 @@ class One2many(_RelationalMulti):
         comodel = records.env[self.comodel_name].with_context(**context)
         inverse = self.inverse_name
         inverse_field = comodel._fields[inverse]
-        get_id = (lambda rec: rec.id) if inverse_field.type == 'many2one' else int
         domain = self.get_domain_list(records) + [(inverse, 'in', records.ids)]
         lines = comodel.search(domain)
 
+        if len(records) == 1:
+            # optimization: all lines have the same value for 'inverse_field',
+            # so we don't need to fetch it from database
+            records.env.cache.insert_missing(records, self, [lines._ids])
+            records.env.cache.insert_missing(lines, inverse_field, itertools.repeat(records.id))
+            return
+
         # group lines by inverse field (without prefetching other fields)
+        get_id = (lambda rec: rec.id) if inverse_field.type == 'many2one' else int
         group = defaultdict(list)
         for line in lines.with_context(prefetch_fields=False):
             # line[inverse] may be a record or an integer