This article was originally posted onmy blog.

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 thename of a field, it'll be parsed as an array! So for an array of emails to send invites; setting name asinvites[] 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.timesdo%><!-- id has to be nil to prevent clashes as all fields would have the same id --><%=form.email_fieldnil,id:nil,placeholder:"Email Address"%><%end%><%=form.submit%><%end%>

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

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

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

defcreateinvited_users=params.require(:invites)pinvited_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!