title: Find instances of model A connected to an instance of B with specific ID when A relates to C through B
body: <p>In LoopBack, I have a model <code>Provider</code>, which has a 'hasMany' relations with a model <code>Customer</code> through model <code>Appointment</code>. Model <code>Customer</code> itself has a 'hasMany' relation back to model <code>Provider</code> through the same model <code>Appointment</code>.</p>
  
  <p>How can I find instances of <code>Customer</code> related to a given <code>Provider</code> whose ID is <code>aderwaze234</code>?</p>