I have a content type called Project. I am linking users to this content type through several user reference fields.

I am also creating related Events (another content type) related to the Project content type pages through the Node Reference URL Widget.

So far so good, but…

In the Event content type I would like to add Attendees, through another user reference field, and limit the list of users to only those referenced in the original Project page. First I tried using a view, but I couldn’t find the proper arguments / filters / relationships. Now I am trying through a custom module with hook_form_alter.

Could I get some advice on an efficient way to do this? Any ideas would be greatly appreciated.

EDIT: I have been working on this, and it seems I am almost there, but not quite. Bare with me as this is my first custom module and my php skills are quite basic. My structure (content type > fields) is as follows:

Project:

• field_users (entity reference)

Event

• field_project (entity reference, prepopulated from Project on event creation)
• field_attendees (entity reference)

Then, in a custom module, I added the following code to modify the Event content type form:

<?php function mymodule_form_alter(&$form, $form_state, $form_id) {   if ($form_id == 'project_event_node_form') {     $id_project=var_export( $form['field_project']['und']['#default_value'][0], TRUE );     $project = node_load($id_project);     $user_ref = array();     foreach($project->field_users as $u){       $user_ref[] = $u;     }     $form['field_attendees']['#options'] = $user_ref;   } } 

But I still see all users, not just those referenced in the Project. Strangely enough, if I do: dpm($user_ref); I get this array, which contains the ids of the users I want to be able to choose from:

array (   0 =>    array (     0 =>      array (       'target_id' => '7',     ),     1 =>      array (       'target_id' => '8',     ),     2 =>      array (       'target_id' => '9',     ),     3 =>      array (       'target_id' => '10',     ),   ), ) 

But somehow I fail to pass that as the options for field_attendees. Any ideas?