title: Mongo DB find by field name with any value
body: <p>I am trying to find all documents that have this field, it doesn't matter what exact value it contains; I am only interested in the existence of it.</p>
  
  <p>Here is an example:</p>
  
  <pre><code>{  
     "payload":{  
        "products":{  
           ...
        },
        "discount":{  
           "type":"1%",
           "value":"1",
           "name":"New Year Discount 1%"
        }
     }
  }
  </code></pre>
  
  <p>I need to target any document that has a field with the name <code>discount</code>. How can I write a query to get such documents? </p>