Read a join table in ruby on rails

As I can read (find, take) a join table created from the contact and group tables, in the migration the join table ContactGroup was created, but I do not see it in the model.

Attachment migration of contact table, group and contact group.

class CreateContactos < ActiveRecord::Migration[5.1]
  def change
    create_table :contactos do |t|
      t.string :nombre
      t.string :numero
      t.string :archivo, default: "** cargado en linea **"

      t.timestamps
    end
  end
end

...

class CreateGrupos < ActiveRecord::Migration[5.1]
  def change
    create_table :grupos do |t|
      t.string :nombre

      t.timestamps
    end
  end
end

...

class CreateJoinTableGrupoContacto < ActiveRecord::Migration[5.1]
  def change
    create_join_table :grupos, :contactos do |t|
      # t.index [:grupo_id, :contacto_id]
      # t.index [:contacto_id, :grupo_id]
    end
  end
end

My versions of ruby and RoR are respectively:

ruby -v

ruby 2.3.3p222 (2016-11-21 revision 56859) [i386-mingw32]

rails -v

Rails 5.1.3

to clone and execute it:

git clone link

cd Contacts and Groups

rails db: migrate

rails s