title: Loopback - delete a record with composite keys
body: <p>I just started using loopback and I have this 'Orders' model:</p>
  
  <pre><code>{
    "name": "Orders",
    "base": "PersistedModel",
    "idInjection": true,
    "options": {
      "validateUpsert": true
    },
    "properties": {
      "Client": {
        "type": "string",
        "id": 1,
        "required": true
      },
      "OrderId": {
        "type": "string",
        "id": 2,
        "required": true
      },
      "OrderName": {
        "type": "string",
        "required": true
      }
    },
    "validations": [],
    "relations": {},
    "acls": [],
    "methods": {}
  }
  </code></pre>
  
  <p>Say I want to delete an order using its composite keys (Client and OrderId). I am using this method but it doesn't delete the record:</p>
  
  <pre><code>var filter = {
                 where: {
                    Client: 'AB123',
                    OrderId: 'ORD1',
                    OrderName: 'Coffee'
                 }
              };
  
  Orders.findOne({filter: filter}, function(err, order){
                  if(err){
                     console.log(err);
                  }else{
                     console.log(order); //prints { Client: 'AB123', OrderId: 'ORD1', OrderName: 'Coffee' }
                     order.destroy();
                  }
                 });
  </code></pre>
  
  <p>I also tried using deleteById() method but it only accepts one id/key. Any workaround here?</p>
  
  <p>Thanks in advance.</p>