I think to begin with, an appropriate change to your data model would be: create table answers_questions ( id int not null autoincrement, question_id int unsigned not null, answer_id int unsigned not null, constraint fk_qid foreign key (question_id) references questions(id), constraint fk_aid foreign key (answer_id) references answers(id), primary key (id) ) ENGINE=InnoDB; In general, it's considered better practice to have primary keys be 'meaningless' and not use them to implement what essentially is a business rule. You can then implement uniqueness on the two foreign keys directly in active record using the built in validation capabilities it provides to ensure that you don't get duplicates. For info on validation, see: http://api.rubyonrails.com/classes/ActiveRecord/Va... You have a choice to put the validation logic in the database or put it in the ActiveRecord models. There are reasons to go either way, but in this case pulling the valdation out of the database and putting it in the ActiveRecord models looks like it may be easier nd give you more flexibility. -- Kevin Bedell http://www.kbedell.com "The future belongs to those who believe in the beauty of their dreams." - Eleanor Roosevelt ----- End forwarded message ----- -- Kevin Bedell http://www.kbedell.com "The future belongs to those who believe in the beauty of their dreams." - Eleanor Roosevelt
on 2006-01-05 06:04
on 2006-01-05 10:38
Hi Kevin, thanks for the advice. I don't believe changing my data model is the right thing here. For a start it's already helped me identify this potential problem :) Also, those rows are already unique by the combination of answer_id and question_id. Adding a new primary key called id is meaningless. An additional problem is that ActiveRecord includes all columns from the link table when joining the linked table, so the id field would collide (the link table id would take preference as I understand). Technically, the primary keys in this instance aren't implementing a business rule. The link table should never include duplicate link records. At the end of the day, I'm trying to do something that violates this rule, which is wrong. Perhaps the best course of action is just to figure out a graceful way to handle it in my code. I don't know how to override the @question.answers.push method, if this were to be the best option. John. http://johnleach.co.uk p.s: Having just tested it, I see that without this compound primary key, rails creates duplicate rows as I suspected.
on 2006-01-05 12:48
I agree that putting an id column in the join table is a bad idea because of the 'clobbering' that will happen. The documentation even mentions this. Not to say that you can't, but you would then have to create a different relationship between the models questions (hm)...(bt) answers_questions (bt)...(hm) answers and answers_questions would become a model I might recommend the following however class Question < AR::Base has_and_belongs_to_many :answers # only add the answer to the question only if the answer is not already associated def add_answer(answer) answers << answer unless answers.include?(answer) end end a = Answer.create(...) q = Question.find(1) q.add_answer(a)
on 2006-01-05 13:39
Hi Chris, thanks, I'll use that method. Do you think ActiveRecord's behaviour is correct? Or should it check for duplicates itself here? Without a compound primary key this might have gone undetected and I think DHH's "single layer of cleverness" post has people believing the db shouldn't be checking this stuff. Should I perhaps file a bug? John. http://johnleach.co.uk  http://www.loudthinking.com/arc/000516.html I'm not suggesting that *is* what DHH is saying though :)