_form.html.haml

.modal-dialog
  .modal-content
    .modal-header
      %button.close{type: 'button', 'aria-hidden' => "true", 'data-dismiss' => 'modal'} ×
      %h4.modal-title Licensee
    .modal-body
      = form_for @licensee, html: { autocomplete: 'off', class: 'form-horizontal' }, remote: true do |f|
        = render 'shared/errors', object: @licensee
        .control-group
          %label.control-label Name
          .controls.controls-row= f.text_field :name, class: 'input-xlarge'
        .control-group
          %label.control-label Locations
          .controls.controls-row= f.collection_select :location_ids, @locations, :id, :code, {include_hidden: false}, multiple: true
        .form-actions
          = f.submit 'SAVE', class: 'btn btn-primary'

Error

ActiveRecord::RecordInvalid (Validation failed: Location has already been taken):
  app/controllers/licensees_controller.rb:41:in `block (2 levels) in update'
  app/controllers/licensees_controller.rb:38:in `update'

licensee.rb

class Licensee < ActiveRecord::Base
  has_many :licensee_locations, dependent: :destroy, autosave: true
  has_many :locations, through: :licensee_locations

  validates :name, presence: true
  validates :name, uniqueness: true
end

licensee_location.rb

class LicenseeLocation < ActiveRecord::Base
  belongs_to :licensee
  belongs_to :location

  validates :licensee, :location, presence: true
  validates :location, uniqueness: true
end

licensees_controller.rb

class LicenseesController < ApplicationController
  before_action :authenticate_user!
  before_action { authorize! }
  before_action { css_select 'admin' }

  def index
    load_licensees
  end

  def new
    respond_to do |format|
      format.js {
        @licensee = Licensee.new
        render_form
      }
    end
  end

  def create
    respond_to do |format|
      format.js {
        @licensee = Licensee.new(safe_params)
        @licensee.save ? render_list : render_form
      }
    end
  end

  def edit
    respond_to do |format|
      format.js {
        find_licensee
        render_form
      }
    end
  end

  def update
    respond_to do |format|
      format.js {
        find_licensee
        @licensee.update(safe_params) ? render_list : render_form
      }
    end
  end

  def destroy
    respond_to do |format|
      format.js {
        find_licensee
        @licensee.destroy
        render_list
      }
    end
  end

  private

  def load_licensees
    @licensees = Licensee.order(:name)
  end

  def find_licensee
    @licensee = Licensee.find(params[:id])
  end

  def load_locations
    @locations = Location.order(:code)
  end

  def render_list
    load_licensees
    render template: 'licensees/list'
  end

  def render_form
    load_locations
    render template: 'licensees/form'
  end

  def safe_params
    params.require(:licensee).permit(:name, location_ids: [])
  end
end