has_many through

has_many_through

# using has_many :through

rails g model mountain name
rails g model climber name
rails g model adventure climber:references mountain:references

class Climber < ApplicationRecord
	has_many :adventures
	has_many :mountains, :through => :adventures
end

class Mountain < ApplicationRecord
	has_many :adventures
	has_many :climbers, :through => :adventures
end

class Adventure < ApplicationRecord
  belongs_to :climber
  belongs_to :mountain
end

$ rake db:create
$ rake db:migrate (or 'rake db:migrate RAILS_ENV=test' for the test environment)
$ rails c
john = Climber.create(name: "John Doe")
udalaitz = Mountain.create(name: "Udalaitz")
john.mountains << udalaitz