Get an array from a Rails form

You still need a fields_for in your view, just use :relationships as the record_name then provide an object.

<%= form_for @account do |f| %>
    <%= f.text_field :name %>

    <% fields_for :relationships, @eligible_parents do |p| %>
        <%= p.check_box "relationships", nil, :value => p.object.id  %>
        <b><%= p.object.name %></b><br/>
    <% end %>

    <%= f.submit "Submit" %>
<% end %>

Documentation here: ActionView::Helpers::FormHelper

If you want to send array of values just use [] in name attributes.In your case just use

<%= f.check_box "relationships", {}, :value => p.id, :name => "relationships[]"   %>

I found this to be the cleanest way...

If you are working with straight data and want to send back an array without using any of these @objects:

<%= form_for :team do |t| %>
  <%= t.fields_for 'people[]', [] do |p| %>
    First Name: <%= p.text_field :first_name %>
    Last Name: <%= p.text_field :last_name %>
  <% end %>
<% end %>

your params data should return like this:

"team" => {
  "people" => [
    {"first_name" => "Michael", "last_name" => "Jordan"},
    {"first_name" => "Steve", "last_name" => "Jobs"},
    {"first_name" => "Barack", "last_name" => "Obama"}
  ]
}