Ruby on Rails Associations - Practical Applications

Rails
1

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

2

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])

3

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
[email protected]

4

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.