title: How to use elemMatch to match nested array
body: <p>I have some data structure like this</p>
  
  <pre><code>{
    a: 1,
    array1: [
      {
        b: 2
        array2: [
          { 
            // this is my target
            c: 3,
            d: 3
          },
          {
            c: 4,
            d: 4
          }          
        ]
      },
      {
        b: 3
        array2: [
          {
            c: 5,
            d: 5
          },
          {
            c: 6,
            d: 6
          }          
        ]
      }
    ]
  }
  </code></pre>
  
  <p>I know use <code>{"array1" : {"$elemMatch" : {"b" : 2} } }</code> to match element of first level array1. But I don't know how to match element <code>{c: 3, d: 3}</code> of array2 of array1.</p>