diff --git a/purchase.py b/purchase.py
index 8093e3b944..552a113216 100644
--- a/purchase.py
+++ b/purchase.py
@@ -3,6 +3,8 @@
 from decimal import Decimal
 from itertools import chain
 
+from sql.functions import CharLength
+
 from trytond import backend
 from trytond.i18n import gettext
 from trytond.model import (
@@ -213,6 +215,11 @@ class PurchaseRequisition(Workflow, ModelSQL, ModelView):
                 [table.state], ['cancelled'],
                 where=table.state == 'cancel'))
 
+    @classmethod
+    def order_number(cls, tables):
+        table, _ = tables[None]
+        return [CharLength(table.number), table.number]
+
     @classmethod
     def default_state(cls):
         return 'draft'
diff --git a/setup.py b/setup.py
index a96c2f7be2..a412c56ced 100755
--- a/setup.py
+++ b/setup.py
@@ -58,7 +58,7 @@ else:
 if local_version:
     version += '+' + '.'.join(local_version)
 
-requires = []
+requires = ['python-sql']
 for dep in info.get('depends', []):
     if not re.match(r'(ir|res)(\W|$)', dep):
         requires.append(get_require_version('trytond_%s' % dep))