Add value_decorator into SequenceField

Allows post processing of the calculated counter value.

docs/changelog.rst

@@ -4,6 +4,7 @@ Changelog
 
 Changes in 0.8
 ==============
+- Updated SequenceFields to allow post processing of the calculated counter value (MongoEngine/mongoengine#141)
 - Added clean method to documents for pre validation data cleaning (MongoEngine/mongoengine#60)
 - Added support setting for read prefrence at a query level (MongoEngine/mongoengine#157)
 - Added _instance to EmbeddedDocuments pointing to the parent (MongoEngine/mongoengine#139)

docs/upgrade.rst

@@ -57,6 +57,13 @@ you will need to declare :attr:`allow_inheritance` in the meta data like so:
         meta = {'allow_inheritance': True}
 
 
+SequenceFields
+--------------
+
+:class:`~mongoengine.fields.SequenceField`s now inherit from `BaseField` to
+allow flexible storage of the calculated value.  As such MIN and MAX settings
+are no longer handled.
+
 0.6 to 0.7
 ==========
 

mongoengine/fields.py

@@ -1329,7 +1329,7 @@ class GeoPointField(BaseField):
             self.error('Both values in point must be float or int')
 
 
-class SequenceField(IntField):
+class SequenceField(BaseField):
     """Provides a sequental counter see:
      http://www.mongodb.org/display/DOCS/Object+IDs#ObjectIDs-SequenceNumbers
 
@@ -1341,15 +1341,26 @@ class SequenceField(IntField):
              cluster of machines, it is easier to create an object ID than have
              global, uniformly increasing sequence numbers.
 
+    Use any callable as `value_decorator` to transform calculated counter into
+    any value suitable for your needs, e.g. string or hexadecimal
+    representation of the default integer counter value.
+
     .. versionadded:: 0.5
+
+    .. versionchanged:: 0.8 added `value_decorator`
     """
+
     _auto_gen = True
+    COLLECTION_NAME = 'mongoengine.counters'
+    VALUE_DECORATOR = int
 
     def __init__(self, collection_name=None, db_alias=None,
-                 sequence_name=None, *args, **kwargs):
+            sequence_name=None, value_decorator=None, *args, **kwargs):
-        self.collection_name = collection_name or 'mongoengine.counters'
+        self.collection_name = collection_name or self.COLLECTION_NAME
         self.db_alias = db_alias or DEFAULT_CONNECTION_NAME
         self.sequence_name = sequence_name
+        self.value_decorator = (callable(value_decorator) and
+            value_decorator or self.VALUE_DECORATOR)
         return super(SequenceField, self).__init__(*args, **kwargs)
 
     def generate(self):
@@ -1364,24 +1375,16 @@ class SequenceField(IntField):
                                              update={"$inc": {"next": 1}},
                                              new=True,
                                              upsert=True)
-        return counter['next']
+        return self.value_decorator(counter['next'])
 
     def __get__(self, instance, owner):
-
+        value = super(SequenceField, self).__get__(instance, owner)
-        if instance is None:
+        if value is None and instance._initialised:
-            return self
-
-        if not instance._data:
-            return
-
-        value = instance._data.get(self.name)
-
-        if not value and instance._initialised:
             value = self.generate()
             instance._data[self.name] = value
             instance._mark_as_changed(self.name)
 
-        return int(value) if value else None
+        return value
 
     def __set__(self, instance, value):
 

tests/test_fields.py

@@ -1,5 +1,8 @@
 # -*- coding: utf-8 -*-
 from __future__ import with_statement
+import sys
+sys.path[0:0] = [""]
+
 import datetime
 import os
 import unittest
@@ -2184,6 +2187,27 @@ class FieldTest(unittest.TestCase):
         c = self.db['mongoengine.counters'].find_one({'_id': 'animal.id'})
         self.assertEqual(c['next'], 10)
 
+    def test_sequence_field_value_decorator(self):
+        class Person(Document):
+            id = SequenceField(primary_key=True, value_decorator=str)
+            name = StringField()
+
+        self.db['mongoengine.counters'].drop()
+        Person.drop_collection()
+
+        for x in xrange(10):
+            p = Person(name="Person %s" % x)
+            p.save()
+
+        c = self.db['mongoengine.counters'].find_one({'_id': 'person.id'})
+        self.assertEqual(c['next'], 10)
+
+        ids = [i.id for i in Person.objects]
+        self.assertEqual(ids, map(str, range(1, 11)))
+
+        c = self.db['mongoengine.counters'].find_one({'_id': 'person.id'})
+        self.assertEqual(c['next'], 10)
+
     def test_generic_embedded_document(self):
         class Car(EmbeddedDocument):
             name = StringField()