diff --git a/openerp/osv/expression.py b/openerp/osv/expression.py
index 2be571e538c..b9e2a396a3a 100644
--- a/openerp/osv/expression.py
+++ b/openerp/osv/expression.py
@@ -994,15 +994,16 @@ class expression(object):
                 rel_table, rel_id1, rel_id2 = column._sql_names(model)
                 #FIXME
                 if operator == 'child_of':
-                    def _rec_convert(ids):
-                        if comodel == model:
-                            return ids
-                        return select_from_where(cr, rel_id1, rel_table, rel_id2, ids, operator)
-
                     ids2 = to_ids(right, comodel, context)
                     dom = child_of_domain('id', ids2, comodel)
                     ids2 = comodel.search(cr, uid, dom, context=context)
-                    push(create_substitution_leaf(leaf, ('id', 'in', _rec_convert(ids2)), model))
+                    if comodel == model:
+                        push(create_substitution_leaf(leaf, ('id', 'in', ids2), model))
+                    else:
+                        subquery = 'SELECT "%s" FROM "%s" WHERE "%s" IN %%s' % (rel_id1, rel_table, rel_id2)
+                        # avoid flattening of argument in to_sql()
+                        subquery = cr.mogrify(subquery, [tuple(ids2)])
+                        push(create_substitution_leaf(leaf, ('id', 'inselect', (subquery, [])), internal=True))
                 else:
                     call_null_m2m = True
                     if right is not False:
@@ -1024,8 +1025,11 @@ class expression(object):
                                 operator = 'in'  # operator changed because ids are directly related to main object
                         else:
                             call_null_m2m = False
-                            m2m_op = 'not in' if operator in NEGATIVE_TERM_OPERATORS else 'in'
-                            push(create_substitution_leaf(leaf, ('id', m2m_op, select_from_where(cr, rel_id1, rel_table, rel_id2, res_ids, operator) or [0]), model))
+                            subop = 'not inselect' if operator in NEGATIVE_TERM_OPERATORS else 'inselect'
+                            subquery = 'SELECT "%s" FROM "%s" WHERE "%s" IN %%s' % (rel_id1, rel_table, rel_id2)
+                            # avoid flattening of argument in to_sql()
+                            subquery = cr.mogrify(subquery, [tuple(filter(None, res_ids))])
+                            push(create_substitution_leaf(leaf, ('id', subop, (subquery, [])), internal=True))
 
                     if call_null_m2m:
                         m2m_op = 'in' if operator in NEGATIVE_TERM_OPERATORS else 'not in'