title: Force JMS Serialiser to output an object keyed by a specific field
body: <p>I have an entity <em>Product</em> with a one-to-many relationship to an entity <em>Property</em>. When I serialise a product instance using the JMS Serialiser I get the following JSON output:</p>
  
  <pre><code>{
      "id": 123,
      "name": "Mankini Thong",
      "properties": [{
          "label": "Minimal size",
          "name": "min_size",
          "value": "S"
      }, {
          "label": "Maximum size",
          "name": "max_size",
          "value": "XXXL"
      }, {
          "label": "colour",
          "name": "Colour",
          "value": "Office Green"
      }]
  }
  </code></pre>
  
  <p>I try to get the serialiser to serialise the properties collection as an object in which a certain field is used as key. For instance, the <em>name</em> field. The desired output is:</p>
  
  <pre><code>{
      "id": 123,
      "name": "Mankini Thong",
      "properties": {
          "min_size": {
              "label": "Minimal size",
              "value": "S"
          }, 
          "max_size": {
              "label": "Maximum size",
              "value": "XXXL"
          }, 
          "colour": {
              "label": "Colour",
              "value": "Office Green"
          }
      }
  }
  </code></pre>
  
  <p>What would be the best approach to achieve this?</p>