Submitting partial forms

Last time, we looked at having a specific button in a form trigger a different action and method than the form’s attributes dictate.

One way this can be used is to submit the form back to the new action, and make adjustments on the server. This is a great way to dynamically load data if you have dependant fields like a country and a province/state:

<%= form_with model: @account do |form| %>
	<!-- ... -->
	
	<div>
		<%= form.label :county %>
		<%= form.select :country, options_for_country(@account.country) %>
		
		<%= form.submit "Update Options", 
			formaction: new_account_path, 
			formmethod: :get %>
	</div>
	
	<div>
		<%= form.label :province %>
		<%= form.select :province, 
			options_for_province(@account.country, @account.province) %>
	</div>
	
	<!-- ... -->
<% end %>

If you click the “Update Options” button, instead of sending the form data to the create action, it will be sent to the new action:

class AccountsController < ApplicationController
	def new
		@account = Account.new(account_params)
	end
	
	def account_params
		# param.require(:account).permit(:county, :province)
		param.fetch(:account, {}).permit(:county, :province)
	end
end

Note that our account_params needs to use fetch instead of require since the params will be blank on first page load.