If you're not familiar with Rails and ActiveRecord in general or with ActiveModel in particular then MongoMapper can be difficult to cope with at times. Even though the docs are fine and in detail some parts seem to be missing that are probably obvious for those coming from Rails. I'm not a Rails guy so I post my experience with many to many associations that might help one or the other save some precious debugging hours.
Take this code:
class User
include MongoMapper::Document
key :name, String, :required => true
key :email, String
key :post_ids, Array
key :favourite_ids, Array
many :posts, :in => :post_ids
many :favourites, :class_name => 'Post', :in => :favourite_ids
end
class Post
include MongoMapper::Document
key :title, String, :required => true
key :body, String
end
u = User.new{:name => "Joe"}
p = Post.new{:title => "My first post", :body => "Hello World"}
u.posts << p
This will work because I told MongoMapper which key collects the many associations for posts. This example is obvious because it's the same as in the docs. If you want to use the favorites you will have to tell MongoMapper which class you want to use. You cannot just make up a name and take it for granted that it'll work. A line like this:
many :favourites, :in => :favourite_ids
won't work because MM will look for a class called Favourite which does not exist. Be explicit and give a class name as in the original example. If you don't Mongomapper will raise an error saying that the BSON object cannot be serialzed into that key. Ver nasty to figure out if you think everything looks great.
Thanks! As someone not familiar about AR, I was really wondering how to do this.