Advice on Simple MongoDB Database Structure

Question

I'm just getting started with MongoDB and Mongoid for Rails and in need of some advice on the right way to design a simple blog database.

I'm currently using the structure below, but I need a way to query all comments written by a given user (the relational db equivalent would be Comment.where('user_id = ?', user_id)).

Is this the right set up, or should I move comments out into their own document, and not embed them in posts (as I would in a relational db schema)?

Appreciate any advice, thanks.

Database Schema

post {
  _id: (object id)
  title: string
  body: string
  user_id: reference
  comments: [
    { _id: (object id), body: string, user_id: reference },
    { _id: (object id), body: string, user_id: reference },
    ...
  ]
}

user {
  _id: (object id)
  name: string
}

In MongoDB, my corresponding models are:

class Post
  include Mongoid::Document
  field :title
  field :body
  embeds_many :comments
  references_one :user
end

class Comment
  include Mongoid::Document
  field :body
  embedded_in :post
  references_one :user
end

class User
  include Mongoid::Document
  field :name
  references_many :posts
end

Once I posted this, StackOverflow threw up [this related post](http://stackoverflow.com/questions/3813975/mongo-db-design-embedding-vs-relationships) which provides a good answer. Still be good to get opinions on the better approach though. — Chris Blunt, Feb 06 '11 at 08:52

score 3 · Accepted Answer · answered Feb 06 '11 at 16:10

3

There's a great article on Mongodb.org about the various choices for modeling comments.

Check out: http://www.mongodb.org/display/DOCS/MongoDB+Data+Modeling+and+Rails#MongoDBDataModelingandRails-ModelingComments

answered Feb 06 '11 at 16:10

jared

1,627
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Thanks. The following quote from that page answers the question perfectly: "It goes without saying that modelling comments in their own collection also facilitates various site-wide aggregations, including displaying the latest, grouping by user, etc." – Chris Blunt Feb 08 '11 at 09:14

score 1 · Answer 2 · answered Feb 09 '11 at 20:56

1

You can use dot-notation in MongoDB to run query filters against embedded documents. In your case, to retrieve all comments by a user, you could simply do:

Post.where("comments.user_id" => myUser.id).all

answered Feb 09 '11 at 20:56

loginx

1,112
9
9

score 0 · Answer 3 · edited Feb 06 '11 at 10:51

0

http://www.mongodb.org/display/DOCS/Dot+Notation+(Reaching+into+Objects)

But not sure how many different options you really need.

Embedded docs or multiple queries or database references are options.

Not sure why this must be asked over and over again.

edited Feb 06 '11 at 10:51

Dogbert

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answered Feb 06 '11 at 09:48

Advice on Simple MongoDB Database Structure

3 Answers3