It seems like a bug. set unique=True make the field be required. and also the spare=True raise Error.

the code is below:

#!/usr/bin/env python
# encoding: utf-8

from mongoengine import Document, connect
from mongoengine import StringField


class Post(Document):
    title = StringField(required=True)
    content = StringField(unique=True)


if __name__ == "__main__":
    connect('test')

    p1 = Post()
    p1.title = "hello"
    p1.save()

and the result show like that:

 $ python mg_models.py 
Traceback (most recent call last):
  File "mg_models.py", line 18, in <module>
    p1.save()
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/document.py", line 224, in save
    self.validate(clean=clean)
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/base/document.py", line 323, in validate
    raise ValidationError(message, errors=errors)
mongoengine.errors.ValidationError: ValidationError (Post:None) (Field is required: ['content'])

this is my env:

 $ uname -a
Linux dp-pc 3.8.0-44-generic #66~precise1-Ubuntu SMP Tue Jul 15 04:01:04 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
 $ python --version
Python 2.7.3

 $ mongod --version
db version v2.6.3
2014-12-04T17:22:12.783+0800 git version: 255f67a66f9603c59380b2a389e386910bbb52cb

In [2]: mongoengine.VERSION
Out[2]: (0, 8, 7)

and also if the follow code:

#!/usr/bin/env python
# encoding: utf-8

from mongoengine import Document, connect
from mongoengine import StringField


class Post(Document):
    title = StringField(required=True)
    content = StringField(unique=True)
    auth = StringField(unique=True, sparse=True)


if __name__ == "__main__":
    connect('test')

    p1 = Post()
    p1.title = "hello"
    p1.save()

give the error:

  $ python mg_models.py 
Traceback (most recent call last):
  File "mg_models.py", line 8, in <module>
    class Post(Document):
  File "mg_models.py", line 11, in Post
    auth = StringField(unique=True, sparse=True)
  File "/usr/local/lib/python2.7/dist-packages/mongoengine/fields.py", line 60, in __init__
    super(StringField, self).__init__(**kwargs)
TypeError: __init__() got an unexpected keyword argument 'sparse'

the doc has this, but I check the source has not such paramters.

Start by running the two Python reproductions against the versions listed in the issue. Inspect mongoengine/document.py, base/document.py, and fields.py, then compare their behavior with the referenced documentation. Done means the unique and sparse field behavior is consistent with the documented API and covered by tests.