How to submit an array using a Rails form

Sometimes an array of data needs to be submitted in an HTML form. A form to send invites to a bunch of users for example. That’d just be a series of email fields; and in the controller; we’d want an array of the emails to iterate over and send invites.

Rails form helpers are set up to have a key and value for each field, so it’s not immediately obvious how to send an array of data. But in Rails, there’s always a way!

If we append [] to the name of a field, it’ll be parsed as an array! So for an array of emails to send invites; setting name as invites[] on all the email fields and gives us an array in the controller.

To generate the form, we can use the following code:

<%= form_with(scope: :invites, url: invite_path) do |form| %>
  <% 3.times do %>
    <!-- id has to be nil to prevent clashes as all fields would have the same id -->
    <%= form.email_field nil, id: nil, placeholder: "Email Address" %>
  <% end %>
  <%= form.submit %>
<% end %>

Passing nil as the first argument to email_field instead of a name gives us the [] we need for an array. The generated markup looks like:

<form action="/invite" accept-charset="UTF-8" data-remote="true" method="post">
  <input type="hidden" name="authenticity_token" value="....">
  <input placeholder="Email Address" type="email" name="invites[]">
  <input placeholder="Email Address" type="email" name="invites[]">
  <input placeholder="Email Address" type="email" name="invites[]">
  <input type="submit" name="commit" value="Save Invites" data-disable-with="Save Invites">
</form>

On the controller, we can use the submitted data as:

def create
  invited_users = params.require(:invites)

  p invited_users
  # => ["a@example.com", "b@example.com", "c@example.com"]
end

To permit an array parameter in a nested form field, we can use the following pattern:

params.require(:product).permit(tags: [])

It’s a little unintuitive but submitting arrays from a form can be a useful tool in your toolbox!