Question

Could not find table ‘groups_users’

<% for group in @groups %>
  <%= check_box_tag "user[group_ids][]", group.id, @user.groups.include?(group) %>
  <%= group.description %>
<% end %>

Answer 1

When using a habtm relationship, you need to create the join table. This is a simple table which stores the relationship by referring to each table via foreign key.

rails g migration create_groups_users_table

class CreateGroupUsersTable < ActiveRecord::Migration
  def change
    create_table :groups_users, :id => false do |t|
      t.integer :group_id
      t.integer :user_id
    end
  end
end

rake db:migrate

Note that the above is for rails 3.1, as it uses the change method. Change to down and up respectively when using rails 3 or lower.

After running the migration, it should work.