Rails 4: How to update a collection_select based on another collection_select through AJAX?

Question

The problem: I need to filter a collection of Units based upon the selection of a collection of Organizations.

Upo

Answer 1

Instead of placing an duplicate organization_id on the projects table you should set the relationship up to go through the Unit model.

class Organization < ActiveRecord::Base
  has_many :units
  has_many :projects, through: :units
end

class Unit < ActiveRecord::Base
  belongs_to :organization
  has_many :projects
end

class Project < ActiveRecord::Base
  belongs_to :unit
  has_one :organizaton, through: :unit
end

This avoids the awkward issue where you have to ensure that a unit and a project have the same organizaton_id.

This also means that you no longer need a input to select the organization when creating a Project - just the unit.