Has_many creation: through entries 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 

Therefore, when I try to connect questions with testing on it, create:

 >> 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"> 

Created 6 SQL queries and 6 records in test_questions. What for?

Answer 1

I created a very simple example that processes 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 entries 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">] >> 

In fact, this is exactly the same as yours, except for TestingQuestion (which you did not show). Does it help?

Answer 2

The first problem might be naming your join table

 testing_questions 

Rails expects the join table name to be a union of two table names in alphabetical order

 question_testings 

Answer 3

If you look at the structure of the table, this is the relationship has_and_belongs_to_many between tables, there is no need to use has_many through the parameter here, since your join table will not have a model (there are no additional columns except identifiers). There is no need to create a model for the join table.

So, I suggest changing your models as follows

 
 class Question < ActiveRecord::Base 
 WEIGHTS = %w(medium hard easy) belongs_to :test 
 has_many :answers, :dependent => :destroy 
 has_and_belongs_to_many :question_testings end 

class Testing < ActiveRecord::Base
belongs_to :student, :foreign_key => 'user_id' belongs_to :subtest has_and_belongs_to_many :question_testings end

and please change the name of your table as question_testings.

Please see this link for more information http://guides.rubyonrails.org/association_basics.html#choosing-between-has-many-through-and-has-and-belongs-to-many

Answer 4

Without looking at the source of Rails or trying, I suggest trying to either remove "has_many: testing_questions" from the Question class, or add has_many ...: through it. Only now Rails only has to do with the join table, but not with the “real” purpose on this side.

The connection table name should not cause any problems.

Answer 5

Your questions array contains 3 elements. When you create a Testing instance and specify to create it with :questions => questions , it will add them to the questions relation, but since this relation happens through another, they must be added to another, as a result, inserted into them twice. Typically, you will have a different model representing the connection table, so you have 3 records inserted in the Questions table, and 3 in the TestingQuestions connection table (has_many: through).

It seems to me that this is a problem in how you defined your TestingQuestions model, which you did not specify. For example, does it point to the same table as the Questions model?