Pre-filter Contacts by Account on Cases

filterContactsFromRelatedAccounts.php

<?php

// file location: /custom/Extension/modules/Contacts/Ext/clients/base/filters/basic/filterContactsFromRelatedAccounts.php

$viewdefs['Contacts']['base']['filter']['basic']['filters'][] = array(
    'id' => 'filterContactsFromRelatedAccount',
    'name' => 'Contacts of Related Account',
    'filter_definition' => array(
        array(
            'account_id' => ''
        )
    ),
    'editable' => true,
    'is_template' => true,
);

record.php

<?php

// file location: /custom/modules/Cases/clients/base/views/record/record.php

$viewdefs['Cases']['base']['view']['record'] = array (
  'buttons' =>
  array(
    // button stuff here
  ),
  'panels' =>
  array(
    // find the panel that has the contact relationship field you want to apply the filter to
    0 =>
    array (
      'name' => 'panel_header',
      'header' => true,
      'fields' =>
      // lots of other fields here, removed for brevity
      array (
      20 =>
        // this is where the magic happens, basically telling the contact field to use 
        // the pre-filter we defined in Contacts module
        array (
          'name' => 'contacts_cases_1_name',
          'label' => 'LBL_CONTACTS_CASES_1_FROM_CONTACTS_TITLE',
          'initial_filter' => 'filterContactsFromRelatedAccount',
          'initial_filter_label' => 'Contacts of Related Account',
          'filter_relate' =>
          array (
            'account_id' => 'account_id',
          ),
        ),
        // end the magic
      ),
    ),
  ),
  'templateMeta' =>
  array (
    'useTabs' => false,
  ),
);