How to create "two-side" many-to-many relationships in Rails?
Suppose we have a photography site. Any author can subscribe to receive updates from any other author. Obviously if author A is subscribed to author B that doesn't mean that B is subscribed to A. So we build models
class Author < ActiveRecord::Base
has_many :subscriptions
has_many :subscribed_by_author, :through => :subscriptions, :source => :subscribed_to
end
class Subscription < ActiveRecord::Base
belongs_to :author
belongs_to :subscribed_to, :class_name => "Author", :foreign_key => "subscribed_to"
end
This way we can use
- some_author.subscribed_by_author -- the list of the authors to whom some_author is subscribed.
- For any subscription we can know both ends (who is subscribed to whom)
But the question is how to get the list of people subscribed to some author using only rails (not using plain SQL) i.e get the answer to :"Who is subscribed to some_author?"
Question: is there any ability in Rails to get the relationship working both sides i.e. not only writing some_author.subscribed_BY_author
but having some_author_subscribed_TO_author
? If there is one, then what is it?
P.S. Obvious solution is to
- Change the database design, adding a column named "direction"
- Create 2 records each time a subscription is created
Add to the author model
has_many :subscribed_BY_author, :through => :subscriptions, :source => :subscribed_to, :conditions => "direction = 'by'"
has_many :subscribed_TO_author, :through => :subscriptions, :source => :subscribed_to, :conditions => "direction = 'to'"
But i w开发者_运维问答onder if there is a solution without changing the database design.
I'd use plain HABTM for something simple like this, but you're going to need a join table no matter what.
create_table :subscriptions do |t|
t.column :author_id, :integer
t.column :subscriber_id, :integer
end
Point Author to it:
class Author < ActiveRecord::Base
has_and_belongs_to_many :subscribers
:class_name => "Author",
:join_table => "subscriptions",
:association_foreign_key => "subscriber_id"
def subscriptions # "subscribers" is already included above
self.subscribers.find(:all, :subscriber_id=>author.id) # hopefully not too
end # much SQL
end
If you're really committed to your method names:
def subscribed_to_author
subscribers
end
def subscribed_by_author(author)
self.subscribers.find(:all, :subscriber_id=>author.id)
end
Create some connections (I'd make SubscriptionsController to be RESTy)
SubscriptionsController < ApplicationController
def create
@author = Author.find(params[:author_id] # author to be subscribed to
@user = current_user # user clicking the "subscribe" button
@author.subscribers << @user # assuming authors should only
@author.save # be able to subscribe themselves
end
end
Display names, or whatever
@author.subscribers.each do |s|
s.name
end
# ...or...and...also...
<%= render :partial => @author.subscribers -%>
<%= render :partial => @author.subscriptions -%>
# Author model
has_many :subscriptions_to, :class_name => "Subscription", :foreign_key => "subscribed_to"
has_many :subscribed_to_author, :through => :subscriptions_to, :source => :author
As far as I know - it works! :)
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