Rails 4 sample for grouping Devices into DeviceGroups (form_for ActiveRecord has_many :through Association)

device_groups_controller.rb

class DeviceGroupsController < ApplicationController

  # ...
  private
  
  def device_group_params
    params.require(:device_group).permit(:title, {:group_ids => []} )
  end
    
  # ...
end

form.html.erb

<%= form_for(@device_group) do |f| %>  
  ...
  <div class="field">
    <%= f.label "Check group members:" %>:<br />
    <%= hidden_field_tag "device_group[device_ids][]", nil %>
    <% Device.all.each do |device| %>
      <%= check_box_tag "device_group[device_ids][]", device.id, @device_group.device_ids.include?(device.id), id: dom_id(device) %>
      <%= label_tag dom_id(device), device.name %>
    <% end %>
  </div>
  ...
<% end %>

models.rb

class Device < ActiveRecord::Base
  has_many :device_groupings, :dependent => :destroy
  has_many :device_groups, :through => :device_groupings
end

class DeviceGroup < ActiveRecord::Base
  has_many :device_groupings, :dependent => :destroy
  has_many :devices, :through => :device_groupings
end

class DeviceGrouping < ActiveRecord::Base
  belongs_to :device_group
  belongs_to :device
end