Autoincrement with MongoEngine

Question

I'm developing a blog engine with Flask and MongoEngine, and I need sequential IDs for my posts.

I need MongoEngine to create a new ID for each new post, so I was thinking of doing something like this:

class Post(Document):
  title = StringField(required=True)
  content = StringField(required=True)
  published_at = datetime.utcnow()
  id = Post.objects.count() + 1

Will this work? is there a better way to do this?

Answer 1

Firstly, you need to understand why you need incremental id's? What do they solve?

Theres no native solution in mongoDB - please read: http://www.mongodb.org/display/DOCS/How+to+Make+an+Auto+Incrementing+Field

As you already have a unique identifier with the pk of the Post, why not use that?

Finally, if I haven't dissuaded you from folly, there is a SequenceField in mongoengine that handles incrementing for you.

Answer 2

Edit: This is an incorrect solution, as others pointed out that this approach causes a race condition. I have only left it here so others would know why this is bad. (multiple clients can access this same object and increment it, resulting in inconsistent results).

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Old answer:

I figured it out.

The Post class looks like this:

class Post(Document):
      title = StringField(required=True)
      content = StringField(required=True)
      published_at = datetime.utcnow()
      ID = IntField(min_value=1)

And in the function that inserts the post, I count the available records and then increment them by 1, like so:

def create_post(title, content):
      Post(title=title, content=content, ID=Post.objects.count() + 1).save()