Deleting only first occurrence from many to many collection

rubyonrails-talk
1

Hello, I am having a little bit problem in deleting duplicate entry of my many to many tables.

In many to many fields delete method is deleting all the occurrence of collection. Say I have (demo example):

  class user < ActiveRecord::Base
has_and_belongs_to_many :cars
end
class car < ActiveRecord::Base
has_and_belongs_to_many :users
end

users and cars are many to many relationship, I have defined my users_cars table. Now user can have repetitive car entry as relation. For example:

Car: A,B,C User: U1,U2,U3

U1=[A,B,C,A,A,A,B]

Which can be implemented using many to many relationship, the way I have implemented. BUT, at the time when I want to delete one of the car entries of user the problem occurs.

Users.cars.delete(car) #deletes all occurrence of car
Users.cars.delete_at(User.cars.find_index(video_card)) #delete_at does not exist

Now how to resolve this?

I have also posted at stackoverslow (http://stackoverflow.com/questions/31507056/deleting-first-occurrence-from-many-to-many-collection-entry-at-rails). Please feel free to answer, and/or upvote.

Thanks.

2

How can a user have the same car multiple times, it doesn't make sense.

Colin

3

Don’t know what your skill level is, but I would suggest checking out RailsCasts, #47 Two Many-to-Many: #47 Two Many-to-Many - RailsCasts Your join table could be called CarOwners, as a point relevant to the RailsCast… Check it out… The general idea is that it is maybe better if you use :has_many :through…

Hope this helps.

4

@colin, certainly I can have a same model car twice :stuck_out_tongue: whatever, it was just a made up scenario. @Elizabeth, railscast helped. I did it with has_many :through. :slight_smile:

Thank you everyone.

5

If it is the model of car (not an actual car) then you should call the model CarModel or similar to avoid confusion. In that case I agree with Elizabeth, use has_many through. Also you might like to consider having a counter in the join table rather than multiple joins, or perhaps that is what you have done already.

Colin

6

@colin, certainly I can have a same model car twice :stuck_out_tongue: whatever, it was just a made up scenario.

If it is the model of car (not an actual car) then you should call the model CarModel or similar to avoid confusion. In that case I agree with Elizabeth, use has_many through. Also you might like to consider having a counter in the join table rather than multiple joins, or perhaps that is what you have done already.

Or possibly even better User has_many cars CarModel has_many cars

So the join model means something in the real world.

Colin