title: Overwriting field on on entity with a default value
body: <p>I have an entity with a field with a value(that can be null). In another table I have the default value. I want to override the entity's value(with the one from the defaults table) if it's null.</p>
  
  <pre><code>products
  +- country_id -+- price -+
  |           1  |   100   |
  |           2  |   NULL  |
  +--------------+---------+
  
  defaults
  +- country_id -+- price -+
  |           1 |      10  |
  |           2 |      99  |
  +-------------+----------+
  
  // this product should load price from defaults
  $product = $productRepository-&gt;findOneBy(['country_id' =&gt; 2]);
  </code></pre>
  
  <p>Is there something is Symfony that will allow me this?</p>
  
  <p>Maybe though constraints?</p>