Rails hidden field undefined method 'merge' error

Question

You should do:

<%= f.hidden_field :service, :value => "test" %>

hidden_field expects a hash as a second argument

You are using a hidden_field instead of a hidden_field_tag. Because you are using the non-_tag version, it is assumed that your controller has already set the value for that attribute on the object that backs the form. For example:

controller:

def new
  ...
  @order.service = "test"
  ...
end</pre>

view:

<%= form_for @order do |f| %>
  <%= f.hidden_field :service %>
  <%= f.submit %>
<% end %>

It works fine in Ruby 1.9 & rails 4

<%= f.hidden_field :service, value: "test" %>

A version with the new syntax for hashes in ruby 1.9:

<%= f.hidden_field :service, value: "test" %>

This also works in Rails 3.2.12:

<%= f.hidden_field :service, :value => "test" %>