Question

I have two models:

class Sentence < ActiveRecord::Base
  attr_accessible :sentence_id, :authority_name #...
end

class Rule < ActiveRecord::Base
  attr_accessible :description, :headline, :note, :sentence_id
end

I would like to know how to create belongs_to :sentence association on Rule which will behave like this pseudo-SQL code:

SELECT * FROM rules 
INNER JOIN sentences ON rules.sentence_id = sentences.sentence_id;

Edit:

I would like get something like that

rule = Rule.find 797 
# we all know how SQL query will look like...
rule.sentence  
# => SELECT * FROM sentences 
     INNER JOIN rules ON rules.sentence_id = sentences.sentence_id 
     WHERE rules.id = 797

Answer 1

First off, is sentence_id the primary key of the sentences table?

If so then you just need to explicitly set that column as the primary key.

class Sentence < ActiveRecord::Base
  set_primary_key :sentence_id
end

class Rule < ActiveRecord::Base
  belongs_to :sentence
end

If sentence_id is not the primary key, then you need to specify it as the “primary key” of the association. I didn’t get a chance to test the code, but it should be like this:

class Rule < ActiveRecord::Base
  belongs_to :sentence, :primary_key => :sentence_id
end