clients_controller.rb

@user = User.new
 @user.user_name = params[:user_name]
 @user.first_name = params[:first_name]
 @user.last_name = params[:last_name]
 @user.email = params[:email]
 @user.password = params[:password]
 @user.password_confirmation = params[:password_confirmation]
 
 @client.user = @user
 
 if @client.save
 # this save will also create and save the user record.
 else
 end

In our context either client details or his contact details may be updated