title: react-admin: custom user filters at list component
body: <p>My backend work with filters is like this (for example) :</p>
  
  <pre><code>filters = {
    "groupOp": "AND",
    "rules": [
      {
        "field": "id",
        "op": "equal",
        "data": 6
      },
      {
        "field": "partnerId",
        "op": "equal",
        "data": 446
      }
    ],
    "groups": [
      {
        "groupOp": "AND",
        "rules": [
          {
            "field": "username",
            "op": "startswith",
            "data": "Alex"
          }
        ],
        "groups": []
      }
    ]
  }
  </code></pre>
  
  <p>It's working fine with persistence filters, but it's not working with user filters that are passed to the list component. For example:</p>
  
  <pre><code>export const OrdersFilter = (props) =&gt; (
  &lt;Filter {...props}&gt;
      &lt;TextInput label="username" source="username" /&gt;
      &lt;TextInput label="partnerId" source="partnerId" /&gt;
  &lt;/Filter&gt;
  );
  </code></pre>
  
  <p>Because it is a key-value filter and I can't understand how I can add additional fields to the user filter field.</p>
  
  <p>Or how I can wrap url changes ('@@router/LOCATION_CHANGE') after action @@redux-form/CHANGE  to modify the original filter which is passed to the url with the  filtred field name :</p>
  
  <pre><code>filter=%7B%22partnerId%22%3A%226%22%7D&amp;order=DESC&amp;page=1&amp;perPage=10'
  to 
  filter={"field": "partnerId","op": "equal","data": 6}&amp;order=DESC&amp;page=1&amp;perPage=10 
  </code></pre>