title: Mongo::Error::OperationFailure Unknown modifier: $pushAll when updating a model (mongoid, angular, rails)
body: <p>I am using angular-8 on the front-end and Rails(5.2) on backend with Mongoid(6.1). </p>
  
  <p>I have a multi-level nested form for model <code>Event</code> that accepts_nested_attributes_for <code>Ticket</code> that in turn accepts_nested_attributes_for <code>Channel</code>.</p>
  
  <p>Whenever I try to update any event and add new <code>Channel</code> to any <code>Ticket</code> with in event. I receive this error:</p>
  
  <blockquote>
    <p>Mongo::Error::OperationFailure (Unknown modifier: $pushAll. Expected a valid update modifier or pipeline-style update specified as an array (9)):</p>
  </blockquote>
  
  <p>I have looked it up and found <a href="https://github.com/Automattic/mongoose/issues/5574" rel="nofollow noreferrer">here</a>, that $pushAll support is deprecated now and we have to <a href="https://github.com/Automattic/mongoose/issues/5670" rel="nofollow noreferrer">Turn on UsePushEach by default</a>.</p>
  
  <p>But I can't figure out <strong>where should I configure this UsePushEach by default settings in Rails</strong>.</p>
  
  <p>Any help would be appreciated. Thanks!</p>