ruby on rails - Accessing a relationship through another relationship

Question

I have three models: Trip, Visit, Park

The relationships are currently set up like:

class Trip < ApplicationRecord
    has_many :visits
end

class Visit < ActiveRecord::Base
    belongs_to :park, counter_cache: true
    belongs_to :trip, optional: true
end

class Park < ActiveRecord::Base
    has_many :visits
end

Basically, If I have a trip, what I want to return is a list of Parks visited on that trip.

I could iterate the visits and grab the parks and deduplicate but there's most likely a more "railsy" way to achieve this that I just don't know about.

Any suggestions?

Many thanks.

question from:https://stackoverflow.com/questions/65876413/accessing-a-relationship-through-another-relationship

Answer 1

You need a "has_many through" relationship.

class Trip < ApplicationRecord
    has_many :visits
    has_many :parks, through: :visits
end

class Visit < ActiveRecord::Base
    belongs_to :park, counter_cache: true
    belongs_to :trip, optional: true
end

class Park < ActiveRecord::Base
    has_many :visits
end

You only have to add has_many :parks, through: :visits in Trip model. Then from Trip you can access to all parks using the parks relation.

https://guides.rubyonrails.org/association_basics.html#the-has-many-through-association