title: TYPO3 TCA display condition (displayCond) with mysql to MM table
body: <p>I have an extension with a main table with a checkbox which should not be available if the item already has a relation through an MM table, the relative TCA :</p>
  
  <pre><code>'checkbox' =&gt; [
    'displayCond' =&gt;'FIELD:uid:!IN:SELECT uid_foreign FROM tx_myext_object_object_mm',
    'exclude' =&gt; 0,
    'label' =&gt; 'checkbox',
    'config' =&gt; [
      'type' =&gt; 'check',
      'items' =&gt; [
        '1' =&gt; [
          '0' =&gt; 'LLL:EXT:lang/locallang_core.xlf:labels.enabled'
        ]
      ],
    'default' =&gt; 0
    ]
  ],
  </code></pre>
  
  <p>can this syntax be corrected or is it impossible (this snippet does not work) </p>