title: Show deep field source in Api Platforms Admin list
body: <p>I am using the newest Api Platform Admin component and I am trying to show on the <code>Ingredients</code> list a name of <code>Category</code> instead of it's API IRI. As you can see below <code>Category.name</code> is empty.
  <a href="https://i.stack.imgur.com/WiuVP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WiuVP.png" alt="Current state of my List in Admin Panel"></a></p>
  
  <p>I've List defined:</p>
  
  <pre class="lang-js prettyprint-override"><code>const ingredientsList = props =&gt; (
      &lt;ListGuesser {...props}&gt;
          &lt;FieldGuesser source="name" /&gt;
          &lt;FieldGuesser source="category" /&gt;
          &lt;TextField source="category.name" /&gt;
          &lt;FieldGuesser source="description" /&gt;
      &lt;/ListGuesser&gt;
  );
  </code></pre>
  
  <p>When I look at my Browser Network card I see a reponse from my API like below.</p>
  
  <pre class="lang-js prettyprint-override"><code>{
     "@context":"\/api\/contexts\/Ingredient",
     "@id":"\/api\/ingredients",
     "@type":"hydra:Collection",
     "hydra:member":[
        {
           "@id":"\/api\/ingredients\/1",
           "@type":"Ingredient",
           "name":"Orange",
           "category":{
              "@id":"\/api\/ingredient_categories\/1",
              "@type":"IngredientCategory",
              "name":"fruits"
           },
           "description":"Orange."
        }
     ],
     "hydra:totalItems":1
  }
  </code></pre>
  
  <p>But when I do a <code>console.log</code> in custom-defined <code>MyTextField</code> that's based on "<a href="https://marmelab.com/react-admin/Fields.html" rel="nofollow noreferrer"><em>Writing your own component</em></a>" part of React-admin documentation in my Console I see:</p>
  
  <pre class="lang-js prettyprint-override"><code>{
     "@id":"/api/ingredients/1",
     "@type":"Ingredient",
     "name":"Orange",
     "category":"/api/ingredient_categories/1",
     "description":"Orange.",
     "id":"/api/ingredients/1"
  }
  </code></pre>
  
  <p>What's going on here? Why the object in <code>console.log</code> is different and how to show <code>Category</code> name instead of IRI in the list?</p>