Using MongoDB with Ruby On Rails and the Mongomapper plugin

Question

Hello, i am currently trying to learn Ruby On Rails as i am a long-time PHP developer so i am building my own community like page. I have came pritty far and have made the user models and suchs using MySQL. But then i heard of MongoDB and looked in to it a little bit more and i find it kinda nice.

So i have set it up and i am using mongomapper for the connection between rails and MongoDB. And i am now using it for the News page on the site. I also have a profile page for every User which includes their own guestbook so other users can come to their profile and write a little message to them.

My thought now is to change the User models from using MySQL to start using MongoDB. I can start by showing how the models for each User is set up.

The user model:

class User < ActiveRecord::Base
            has_one :guestbook, :class_name => "User::Guestbook"

The Guestbook model model:

class User::Guestbook < ActiveRecord::Base
  belongs_to :user
  has_many :posts, :class_name => "User::Guestbook::Posts", :foreign_key => "user_id"

And then the Guestbook posts model:

class User::Guestbook::Posts < ActiveRecord::Base
  belongs_to :guestbook, :class_name => "User::Guestbook"

I have divided it like this for my own convenience but now when i am going to try to migrate to MongoDB i dont know how to make the tables. I would like to have one table for each user and in that table a "column" for all the guestbook entries since MongoDB can have a EmbeddedDocument. I would like to do this so i just have one Table for each user and not like now when i have three tables just to be able to have a guestbook.

So my thought is to have it like this:

The user model:

class User
  include MongoMapper::Document
  one :guestbook, :class_name => "User::Guestbook"

The Guestbook model model:

class User::Guestbook
  include MongoMapper::EmbeddedDocument
  belongs_to :user
  many :posts, :class_name => "User::Guestbook::Posts", :foreign_key => "user_id"

And then the Guestbook posts model:

class User::Guestbook::Posts
  include MongoMapper::EmbeddedDocument
  belongs_to :guestbook, :class_name => "User::Guestbook"

But then i can think of one problem.. That when i just want to fetch the user information like a nickname and a birthdate then it will have to fetch all the users guestbook posts. And if each user has like a thousand posts in the guestbook it will get really much to fetch for the system. Or am i wrong? Do you think i should do it any other way?

Thanks in advance and sorry if i am hard to understand but i am not so educated in the english language :)

Answer 1

If you are going to have thousands of guestbook entries then it would probably be a good idea to make that a separate collection (for exactly the reason that you've stated).

Answer 2

No you don't have to fetch all the guestbook entries if you fetch a user, this is the mongo query for and MongoMapper shouldn't work so much differently (For myself, I'm using Mongoid):

db.users.find({_id: '21314'}, {guestbook: 0})
                // instead of {guestbook: 1} which would return only the guestbook

Something to be aware of, MongoDB still has a limit of 4mb for a single document but that should be tens of thousands of guestbook entries. You might as well push older ones into a sort of archive.