Ruby on Rails: Add a check box for each value in database

I am trying to display checkboxes on a form that show all of the contacts for a user, so that the user can then select the contacts required. I have done this on the form so far, which displays the checkboxes but with no value:

    <% @contacts.each do |contact| %>
        <% if contact.user.id == User.find(session[:user_id]).id %>
            <%= f.check_box :to %>
        <% end %>
    <% end %>

But when I try to change the line <%= f.check_box :to %> to <%= f.check_box :to, contact.email %> to show the email address of the contact I get the error:

undefined method `merge' for "email@123.com":String

Is there a way around this? I have looked at multiple posts before and tried various options but none seem to help, including:

Ruby on rails f.label for a check box http://ift.tt/1xz2qH5 How to add a checkbox for each row in Rails 3.2 index page?