title: Easyadmin 3: field is not a Doctrine association, so it cannot be used as an association field
body: <p>so this is the situation:
  I have an entity ProductWhisky, which is a subclass of ProductAbstract, in which I refer to the Product entity (One-to-One). Product has many-to-one association to ProductProducer. I'm using Easyadmin 3 to edit this ProductWhisky.</p>
  <p>So this is a part of my ProductWhiskyCrudController:</p>
  <pre><code>public function configureFields(string $pageName): iterable
  {
      // more fields
      yield AssociationField::new('productProducer', 'Producent');
      // even more fields
  }
  </code></pre>
  <p>This is my ProductWhiskyEntity:</p>
  <pre><code>class ProductWhisky extends ProductAbstract
  {
      use ProductTrait;
      //props and other methods specific to ProductWhisky
  }
  
  </code></pre>
  <p>ProductTrait</p>
  <pre><code>trait ProductTrait
  {
      /**
       * @ORM\OneToOne(targetEntity=&quot;App\Entity\Product&quot;, cascade={&quot;persist&quot;, &quot;remove&quot;})
       * @ORM\JoinColumn(nullable=false)
       */
      private $product;
  
      public function getProduct(): ?Product
      {
          return $this-&gt;product;
      }
  
      public function setProduct(Product $product): self
      {
          $this-&gt;product = $product;
  
          return $this;
      }
  
      /**
       * @return ProductProducer|null
       */
      public function getProductProducer(): ?ProductProducer
      {
          if ($this-&gt;getProduct()) {
              return $this-&gt;getProduct()-&gt;getProducer();
          }
          return null;
      }
  </code></pre>
  <p>Product entity:</p>
  <pre><code>class Product
  {
      // more props
      /**
       * @ORM\ManyToOne(targetEntity=&quot;App\Entity\ProductProducer&quot;, inversedBy=&quot;products&quot;, cascade={&quot;persist&quot;})
       * @ORM\JoinColumn(nullable=false)
       */
      private $producer;
      
      // more props and methods
  
      public function getProducer(): ?ProductProducer
      {
          return $this-&gt;producer;
      }
  
      public function setProducer(?ProductProducer $producer): self
      {
          $this-&gt;producer = $producer;
  
          return $this;
      }
  
  </code></pre>
  <p>So, when I call the ProductWhiskyCrudController, I always get the message <code>The &quot;productProducer&quot; field is not a Doctrine association, so it cannot be used as an association field.</code></p>
  <p>In EasyAdmin 2, I did this with this:</p>
  <pre><code>      form:
          fields:
            - property: 'productProducer'
              label: 'Producent'
              type: entity
              type_options: {class: 'App\Entity\ProductProducer', required: true}
  
  </code></pre>
  <p>Any idea how I could fix this?</p>
  <p>All the best
  Tim</p>