diff --git a/openerp/addons/base/ir/ir_model.py b/openerp/addons/base/ir/ir_model.py
index 966f5e29077..8796f892960 100644
--- a/openerp/addons/base/ir/ir_model.py
+++ b/openerp/addons/base/ir/ir_model.py
@@ -151,7 +151,7 @@ class ir_model(osv.osv):
             if result and result[0] == 'v':
                 cr.execute('DROP view %s' % (model_pool._table,))
             elif result and result[0] == 'r':
-                cr.execute('DROP TABLE %s' % (model_pool._table,))
+                cr.execute('DROP TABLE %s CASCADE' % (model_pool._table,))
         return True
 
     def unlink(self, cr, user, ids, context=None):
diff --git a/openerp/osv/orm.py b/openerp/osv/orm.py
index fc381695fef..1f2823b7919 100644
--- a/openerp/osv/orm.py
+++ b/openerp/osv/orm.py
@@ -3385,7 +3385,10 @@ class BaseModel(object):
 
     def _m2m_raise_or_create_relation(self, cr, f):
         m2m_tbl, col1, col2 = f._sql_names(self)
-        self._save_relation_table(cr, m2m_tbl)
+        # do not create relations for custom fields as they do not belong to a module
+        # they will be automatically removed when dropping the corresponding ir.model.field
+        if not f.string.startswith('x_'):
+            self._save_relation_table(cr, m2m_tbl)
         cr.execute("SELECT relname FROM pg_class WHERE relkind IN ('r','v') AND relname=%s", (m2m_tbl,))
         if not cr.dictfetchall():
             if not self.pool.get(f._obj):