Question

Given the following tables in ActiveRecord:

• authors
• sites
• articles

I don’t know how to express that an author is paid a different amount depending on the publication, but that authors working for the same publication have different rates:

• John publishes an article in Foo for $300
• John publishes an article in Bar for $350
• John publishes an article in Baz for $400
• Dick publishes an article in Foo for $250
• Dick publishes an article in Bar for $400

etc.

What kind of relationship am I trying to describe?

At the moment I’ve got a ‘rates’ table with author_id, site_id and amount columns. Given publication.id and author.id, I derive the cost of the article with

cost = Rate.find(:first, :conditions => ['author_id = ? and site_id = ?', author.id, site.id]).rate 

That works, but I’m not sure it’s the best way, and I’m not sure how to make sure I don’t end up with ‘John’ having two rates for ‘Baz.’

I don’t think I want code so much as I want someone to say ‘Oh, that’s a … relationship’ so I can get a grip on what I’m Googleing for.

Answer 1

Its a has and belongs to many with a rich join table.

class Author   has_many :publications, :through => :rates end  class Publication   has_many :authors, :through => :rates end  class Rate #rich join table   belongs_to :author   belongs_to :publication end 

And you can then simplify your finding like this:

@author.rates.find_by_site_id(123) 

Plus you get direct access accross the join table

@author.publications @publication.authors