Filtering based on a has_and_belongs_to_many association (See related posts)

To create an indexed view of either all of the items (in a normal index view), or filtered based on an association, you can do the following. For example, if you have a many to many relationship between "users" and "tags", and you wish an index view of users filtered on whether they contain a particular tag, you can do the following:

In the tag index view:

<h1>Listing tags</h1>

<table>
  <tr>
    <th>Name</th>
  </tr>

<% for tag in @tags %>
  <tr>
    <td><%=h tag.name %></td>
    <td><%= link_to 'Users', user_path(:tag_id=>tag) %></td>
    <td><%= link_to 'Show', tag_path(tag) %></td>
    <td><%= link_to 'Edit', edit_tag_path(tag) %></td>
    <td><%= link_to 'Destroy', tag_path(tag), :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
  <% end %>
</table>
<br />
<%= link_to 'New tag', new_tag_path %>

Note that

user_path(:tag_id=>tag)

translates to

<a href="/users?tag_id=1">Users</a>

.

In the controller for users, modify the index and change the standard @users=User.find(:all) to

    @tag=Tag.find(params[:tag_id]) if params[:tag_id]
    @users = @tag.users || throw rescue User.find(:all)

The

throw rescue

was because the @tag.users could throw an exception as well as having a nil value, and this is DRYer than coding it as

    @users = @tag.users || User.find(:all) rescue User.find(:all)

Full details at Displaying a subset of items from an association.