MongoEngine/mongoengine — pull request #519 by boostbob (2013-11-20 10:55:09+00:00)
Title: add pluck function for queryset

tests case added.

```py
def test_pluck(self):
class Post(Document):
title = StringField()

Post.drop_collection()

for i in xrange(10):
Post(title="Post %s" % i).save()

result = ["Post %s" % i for i in xrange(10)]
self.assertEqual(Post.objects.pluck('title'), result)
```

examples:

```py
#coding:utf-8

from mongoengine import *

connect('mongoengine_test')

class PluckUser(Document):
name = StringField(max_length=50)

PluckUser(name='boostbob').save()
PluckUser(name='debugger').save()
PluckUser(name='zhangsan').save()
PluckUser(name='wangwu').save()

print PluckUser.objects(name__in=['boostbob', 'debugger']).pluck('name')
#[u'boostbob', u'debugger']

PluckUser.objects.delete()
```