title: How do I filter a collection for a null foreign key or object relation in BackAnd?
body: <p>I'd like to filter a collection for a null foreign key or object relation in BackAnd.</p>
  
  <pre><code>filter = [ 
    {
      "fieldName": "Parent",
      "operator": "empty",    
      "value": ""
    }
  ]
  </code></pre>
  
  <p>Here's my table/object definition:</p>
  
  <pre><code>{
    "name": "Tree",
    "fields": {
      "Title": {
        "type": "string"
      },
      "Description": {
        "type": "string"
      },
      "Parent": {
        "object": "Certifications"
      },
      "Children": {
        "collection": "Certifications",
        "via": "Parent"
      }
    }
  }
  </code></pre>
  
  <p>When I try the filter above, I get this error:</p>
  
  <pre><code>The field "FK_Tree_Tree_Parent" is a relation field. 
  To filter relation fields please use the operator "in"
  </code></pre>
  
  <p>This just returns all of the values in the table:</p>
  
  <pre><code>filter = [ 
    {
      "fieldName": "Parent",
      "operator": "in",    
      "value": ""
    }
  ]
  </code></pre>
  
  <p>Is it possible to get back the records that don't have a parent assigned to them? </p>