Before mongoid 2.0.0.rc1, there is no default support for many to many association. So we use join document (aka join table in relational database) to implement the many to many association.
For example, we have two documents users and accounts, one user has many accounts and one account contains many users, to establish the many to many relationship between users and accounts, we create a new document named user_accounts, the document looks like
{'_id': '4d76d3a70bdb822d08000001', 'user_id': BSON::ObjectId('4d76d3b80bdb822d080015b3'), 'account_id': BSON::ObjectId('4d76d3b90bdb822d080015b7')}
and the models are defined as follows
class User
include Mongoid::Document
references_many :user_accounts
end
class Account
include Mongoid::Document
references_many :user_accounts
end
class UserAccount
include Mongoid::Document
referenced_in :user
referenced_in :account
end
Are you familiar with it, it's what activerecord did for many to many association.
I'm glad that mongoid began to support many to many association after mongoid 2.0.0.rc1, the new syntax is "referenes_and_referenced_in_many".
class User
include Mongoid::Document
references_and_referenced_in_many :accounts
end
class Account
include Mongoid::Document
references_and_referenced_in_many :users
end
We don't need the join document any more. The implementation of mongoid is different with activerecord, it uses array attribute to store the relationship at both sides. Like
These are user documents
{'_id': '4d76d3a90bdb822d08000009', account_ids: [BSON::ObjectId('4d76d3aa0bdb822d0800001b'), BSON::ObjectId('4d76d3aa0bdb822d0800001d'), BSON::ObjectId('4d76d3aa0bdb822d08000017')]}
{'_id': '4d76d3a80bdb822d08000005', account_ids: [BSON::ObjectId('4d76d3aa0bdb822d08000017'), BSON::ObjectId('4d76d3a90bdb822d08000015')]}
And these are account documents
{'_id': '4d76d3aa0bdb822d08000017', user_ids: [BSON::ObjectId('4d76d3a80bdb822d08000005'), BSON::ObjectId('4d76d3a90bdb822d08000009'), BSON::ObjectId('4d76d3a90bdb822d0800000d')]}
{'_id': '4d76d3aa0bdb822d0800001b', user_ids: [SON::ObjectId('4d76d3a90bdb822d08000009'), BSON::ObjectId('4d76d3a90bdb822d08000011')]}
As mongodb support the Array type, it is really easy to maintain the many to many relationship.
Btw, if you use
references_many :name, :stored_as => :array
before, you will receive a runtime error. You should use
references_and_referenced_in_many :name
instead.
