title: EntityType as an input field?
body: <p>I'm trying to create a form with a CollectionType field containing an email field (which is one of two fields of an entity) which I want to add and remove as I wish.</p>
  
  <p>Here's the scenario:</p>
  
  <p>I have three entities: <code>User</code>, <code>Cluster</code> and the relationship between both called <code>ClusterAdministrator</code>.</p>
  
  <p>Entity <code>User</code> has plenty of all-around attributes (id, name, email, etc). Entity <code>Cluster</code> has a lot of attributes too, and one of them is <code>administrators</code>, which is an ArrayCollection. Entity <code>ClusterAdministrator</code> has 3 attributes: id, user and cluster.</p>
  
  <p>My BD has around 50K users. If I add an CollectionType field of a UserType, and my UserType is a form with <code>user</code> as a normal EntityType, app will crash because it takes more than 30seconds to render such a huge select box. If I my UserType is a form with <code>user</code> as an EmailType, the form is well rendered (see <a href="https://imgur.com/a/cUFcP" rel="nofollow noreferrer">here</a>), but when I click save I get the error:</p>
  
  <blockquote>
    <p>Expected argument of type "App\AppBundle\Entity\User", "string" given</p>
  </blockquote>
  
  <p>How do I make this work just by showing the email field?</p>
  
  <p>Sorry guys, I'm quite newbie to Symfony. I'll appreciate all of your precious help.</p>