# Ruby on Rails Associations - Practical Applications **URL:** \ **Category:** Rails\ **Created:** [May 10, 2010, 10:57am UTC](https://www.ruby-forum.com/t/ruby-on-rails-associations-practical-applications/188327 "2010-05-10T10:57:12Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![cam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/bbe5ce/32.png) [@cam](https://www.ruby-forum.com/u/cam)\ **Post date:** [May 10, 2010, 10:57am UTC](https://www.ruby-forum.com/t/ruby-on-rails-associations-practical-applications/188327/1 "2010-05-10T10:57:12Z")
Hey All, Forgive me for posting a question that may be very elementary. I’m building an application and am a bit stuck on how to work out the associations. It goes like this: User :has\_many vehicles User :has\_many logs Vehicle :has\_many logs User -\> Vehicles -\> Logs The Vehicles have a user\_id attribute. The Logs have user\_id and vehicle\_id attributes. ## I am using restful authentication and am able to associate the vehicles with the users al a ## def create @vehicle = current\_user.vehicle.build (params[:vehicle]) … I can’t figure out how I should do it for the log. Thanks again, and sorry for my lack of understanding of how rails works. Cheers, Cam Peterson ---
**Author:** ![cam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/bbe5ce/32.png) [@cam](https://www.ruby-forum.com/u/cam)\ **Post date:** [May 10, 2010, 11:14am UTC](https://www.ruby-forum.com/t/ruby-on-rails-associations-practical-applications/188327/2 "2010-05-10T11:14:20Z")
Cam wrote: > Hey All, > Forgive me for posting a question that may be very elementary. > > I’m building an application and am a bit stuck on how to work out the > associations. It goes like this: > > User :has\_many vehicles > User :has\_many logs > > Vehicle :has\_many logs > > User -\> Vehicles -\> Logs > > The Vehicles have a user\_id attribute. > The Logs have user\_id and vehicle\_id attributes. > > ## I am using restful authentication and am able to associate the vehicles with the users al a > > ## def create @vehicle = current\_user.vehicle.build (params[:vehicle]) … > > I can’t figure out how I should do it for the log. > Thanks again, and sorry for my lack of understanding of how rails > works. > > Cheers, > > Cam Peterson In User model write has\_many :logs has\_many :vehicles In Vehicle model write has\_many :logs In your logs model write belongs\_to :user belongs\_to :vehicle In your vehicle model write belongs\_to :user def create @vehicle = current\_user.vehicle.create (params[:vehicle]) ---
**Author:** ![cam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/bbe5ce/32.png) [@cam](https://www.ruby-forum.com/u/cam)\ **Post date:** [May 10, 2010, 4:35pm UTC](https://www.ruby-forum.com/t/ruby-on-rails-associations-practical-applications/188327/3 "2010-05-10T16:35:21Z")
Cam wrote: > Hey All, > Forgive me for posting a question that may be very elementary. > > I’m building an application and am a bit stuck on how to work out the > associations. It goes like this: > > User :has\_many vehicles > User :has\_many logs > > Vehicle :has\_many logs > > User → Vehicles → Logs > > The Vehicles have a user\_id attribute. > The Logs have user\_id and vehicle\_id attributes. > > ## I am using restful authentication and am able to associate the vehicles with the users al a > > ## def create @vehicle = current\_user.vehicle.build (params[:vehicle]) … > > I can’t figure out how I should do it for the log. > Thanks again, and sorry for my lack of understanding of how rails > works. You probably want to use has\_many :through. Have you read the Rails guide on associations? > Cheers, > > Cam Peterson ## Best, Marnen Laibow-Koser [http://www.marnen.org](http://www.marnen.org) removed\_email\_address@domain.invalid ---
**Author:** ![cam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/bbe5ce/32.png) [@cam](https://www.ruby-forum.com/u/cam)\ **Post date:** [May 10, 2010, 10:13pm UTC](https://www.ruby-forum.com/t/ruby-on-rails-associations-practical-applications/188327/4 "2010-05-10T22:13:54Z")
Cam wrote: > I’m building an application and am a bit stuck on how to work out the > associations. It goes like this: > > User :has\_many vehicles > User :has\_many logs > > Vehicle :has\_many logs > > User -\> Vehicles -\> Logs > > The Vehicles have a user\_id attribute. > The Logs have user\_id and vehicle\_id attributes. I see two possible scenarios here. Neither is wrong, They depend on behavior you want. 1. Users have many vehicles and vehicles have many logs. 2. Users have many vehicles through logs. In scenario (1) you would need two one-to-many associations: User has\_many :vehicles Vehicle belongs\_to :user has\_many :logs Log belongs\_to :user Logs would now indirectly belong to a user because vehicles belong to users. Therefore Log would have only a foreign key to vehicle, but no foreign key to user since that would be redundant. Find the User for a Log with: user = my\_log.vehicle.user In scenario (2) there would be a many-to-many association between User and Vehicle with Log joining them. User has\_many :logs has\_many :vehicles, :through =\> :logs Vehicle: has\_many :logs has\_many :users, :through =\> :logs Log belongs\_to :user belongs\_to :vehicle This scenario could be read as Users have many vehicles and vehicles can belong to many users, where logs track the association and can store information pertaining to a specific vehicle belonging to a specific user. I’m guessing this is not the scenario you want, but this pattern certainly has its uses.