Creating has_many :through records 2x times

Question

I have models

class Question < ActiveRecord::Base
  WEIGHTS = %w(medium hard easy)

  belongs_to :test
  has_many :answers, :dependent => :destroy
  has_many :testing_questions
end

class Testing < ActiveRecord::Base
  belongs_to :student, :foreign_key => 'user_id'
  belongs_to :subtest
  has_many :testing_questions, :dependent => :destroy
  has_many :questions, :through => :testing_questions
end

So when I try to bind questions to testing on it's creation:

>> questions = Question.all
...
>> questions.count
=> 3
>> testing = Testing.create(:user_id => 3, :subtest_id => 1, :questions => questions)
  Testing Columns (0.9ms)   SHOW FIELDS FROM `testings`                              
  SQL (0.1ms)   BEGIN                                                                
  SQL (0.1ms)   COMMIT                                                               
  SQL (0.1ms)   BEGIN                                                                
  Testing Create (0.3ms)   INSERT INTO `testings` (`created_at`, `updated_at`, `user_id`, `subtest_id`) VALUES('2010-05-18 00:53:05', '2010-05-18 00:53:05', 3, 1)                                                                                                                                                        
  TestingQuestion Columns (0.9ms)   SHOW FIELDS FROM `testing_questions`                                                                                     
  TestingQuestion Create (0.3ms)   INSERT INTO `testing_questions` (`question_id`, `created_at`, `updated_at`, `testing_id`) VALUES(1, '2010-05-18 00:53:05', '2010-05-18 00:53:05', 31)                                                                                                                                  
  TestingQuestion Create (0.4ms)   INSERT INTO `testing_questions` (`question_id`, `created_at`, `updated_at`, `testing_id`) VALUES(2, '2010-05-18 00:53:05', '2010-05-18 00:53:05', 31)                                                                                                                                  
  TestingQuestion Create (0.3ms)   INSERT INTO `testing_questions` (`question_id`, `created_at`, `updated_at`, `testing_id`) VALUES(3, '2010-05-18 00:53:05', '2010-05-18 00:53:05', 31)                                                                                                                                  
  TestingQuestion Create (0.3ms)   INSERT INTO `testing_questions` (`question_id`, `created_at`, `updated_at`, `testing_id`) VALUES(1, '2010-05-18 00:53:05', '2010-05-18 00:53:05', 31)                                                                                                                                  
  TestingQuestion Create (0.3ms)   INSERT INTO `testing_questions` (`question_id`, `created_at`, `updated_at`, `testing_id`) VALUES(2, '2010-05-18 00:53:05', '2010-05-18 00:53:05', 31)                                                                                                                                  
  TestingQuestion Create (0.3ms)   INSERT INTO `testing_questions` (`question_id`, `created_at`, `updated_at`, `testing_id`) VALUES(3, '2010-05-18 00:53:05', '2010-05-18 00:53:05', 31)                                                                                                                                  
  SQL (90.2ms)   COMMIT                                                                                                                                      
=> #<Testing id: 31, subtest_id: 1, user_id: 3, created_at: "2010-05-18 00:53:05", updated_at: "2010-05-18 00:53:05">

There are 6 SQL queries and 6 records in testing_questions are created. Why?

Answer 1

A first issue could be the naming of your join table

testing_questions

Rails expects the join table’s name to be a concatenation of the two table names in alphabetical order

question_testings

Answer 2

Without looking at the Rails source or trying around, I'd suggest to try either removing the "has_many :testing_questions" from the Question class or add a has_many ... :through there. Just now Rails just has a relation to the join table, but not to the "real" target from that side.

The name of the join table shouldn't cause any problems here.

Answer 3

I have created a very simple example that handles your example:

class Question < ActiveRecord::Base
  has_many :testing_questions
end

class Testing < ActiveRecord::Base
  has_many :testing_questions
  has_many :questions, :through => :testing_questions
end

class TestingQuestion < ActiveRecord::Base
  belongs_to :question
  belongs_to :testing
end

.. and then i can just do the following, and no duplicate records are created:

Loading development environment (Rails 2.3.5)
>> q1 = Question.new
=> #<Question id: nil, title: nil, ask: nil, created_at: nil, updated_at: nil>
>> q1.title = "Dit is de eerste vraag"
=> "Dit is de eerste vraag"
>> q2 = Question.new
=> #<Question id: nil, title: nil, ask: nil, created_at: nil, updated_at: nil>
>> q2.title = "Dit is de tweede vraag"
=> "Dit is de tweede vraag"
>> q1.save
=> true
>> q2.save
=> true
>> tt = Testing.new
=> #<Testing id: nil, name: nil, description: nil, action: nil, created_at: nil, updated_at: nil>
>> tt.questions
=> []
>> tt.name = "Test1"
=> "Test1"
>> tt.questions << q1
=> [#<Question id: 1, title: "Dit is de eerste vraag", ask: nil, created_at:   "2010-05-18 19:40:54", updated_at: "2010-05-18 19:40:54">]
>> tt.questions << q2
=> [#<Question id: 1, title: "Dit is de eerste vraag", ask: nil, created_at: "2010-05-18 19:40:54", updated_at: "2010-05-18 19:40:54">, #<Question id: 2, title: "Dit is de tweede vraag", ask: nil, created_at: "2010-05-18 19:40:59", updated_at: "2010-05-18 19:40:59">]
>> tt.testing_questions
=> []
>> tt.save
=> true
>> tt.testing_questions
=> [#<TestingQuestion id: 1, question_id: 1, testing_id: 1, extra_info: nil, created_at: "2010-05-18 19:41:43", updated_at: "2010-05-18 19:41:43">, #<TestingQuestion id: 2, question_id: 2, testing_id: 1, extra_info: nil, created_at: "2010-05-18 19:41:43", updated_at: "2010-05-18 19:41:43">]
>>

Actually that is completely the same as you have, except for the TestingQuestion (which you didn't show). Does that help?