[FIX] models: avoid prefetch hell when reading fields

The method `read()` may be very slow when reading relational fields and
computed fields, because the computed fields can be computed on a recordset
that is larger than expected.

The issue occurs on model 'res.partner' when reading fields 'child_ids' and
'purchase_order_count', for instance.  Suppose we read those two fields on a
partner with 1000 contacts.  First, the one2many field is read from the
database and stored to the cache; the latter adds the value ids to the
prefetching of 'res.partner'.  Then, the fields are fetched from the cache.
When 'purchase_order_count' is accessed on the partner, the field is computed
on all its children as well...

closes odoo/odoo#29867

odoo/addons/base/tests/test_api.py

@@ -358,6 +358,31 @@ class TestAPI(common.TransactionCase):
         same_prefetch(empty, empty.bank_ids)
         same_prefetch(empty, empty.category_id)
 
+    @mute_logger('odoo.models')
+    def test_60_prefetch_read(self):
+        """ Check that reading a field computes it on self only. """
+        Partner = self.env['res.partner']
+        field = type(Partner).company_type
+        self.assertTrue(field.compute and not field.store)
+
+        partner1 = Partner.create({'name': 'Foo'})
+        partner2 = Partner.create({'name': 'Bar', 'parent_id': partner1.id})
+        self.assertEqual(partner1.child_ids, partner2)
+
+        # reading partner1 should not prefetch 'company_type' on partner2
+        self.env.clear()
+        partner1 = partner1.with_prefetch()
+        partner1.read(['company_type'])
+        self.assertIn('company_type', partner1._cache)
+        self.assertNotIn('company_type', partner2._cache)
+
+        # reading partner1 should not prefetch 'company_type' on partner2
+        self.env.clear()
+        partner1 = partner1.with_prefetch()
+        partner1.read(['child_ids', 'company_type'])
+        self.assertIn('company_type', partner1._cache)
+        self.assertNotIn('company_type', partner2._cache)
+
     @mute_logger('odoo.models')
     def test_70_one(self):
         """ Check method one(). """

odoo/models.py

@@ -3019,17 +3019,19 @@ class BaseModel(object):
 
         # retrieve results from records; this takes values from the cache and
         # computes remaining fields
-        result = []
-        name_fields = [(name, self._fields[name]) for name in (stored + inherited + computed)]
+        data = {record: {'id': record.id} for record in self}
+        missing = set()
         use_name_get = (load == '_classic_read')
-        for record in self:
-            try:
-                values = {'id': record.id}
-                for name, field in name_fields:
-                    values[name] = field.convert_to_read(record[name], record, use_name_get)
-                result.append(values)
-            except MissingError:
-                pass
+        for name in (stored + inherited + computed):
+            convert = self._fields[name].convert_to_read
+            # read every field with prefetching limited to self; this avoids
+            # computing fields on a larger recordset than self
+            for record in self.with_prefetch():
+                try:
+                    data[record][name] = convert(record[name], record, use_name_get)
+                except MissingError:
+                    missing.add(record)
+        result = [data[record] for record in self if record not in missing]
 
         return result