title: Get array of partial model from sub document array (MongoDB C# Driver)
body: <p>I'm trying to receive new array with only sub fields filled using MongoDB C# Driver. For example I have the following document: </p>
  
  <pre><code>{
      "_id" : "fca739d0-cddd-4762-b680-597d2996404b",
      "Status" : 1,
      "AccountId" : "1112",
      "Timestamp" : ISODate("2016-04-27T13:46:01.888Z"),
      "CartItems" : [ 
          {
              "ProductId" : "222",
              "Price" : 100,
              "ShippingPrice" : 20,
              "Quantity" : 3
          }, 
          {
              "ProductId" : "504",
              "Price" : 200,
              "ShippingPrice" : 20,
              "Quantity" : 2
          }, 
          {
              "ProductId" : "504",
              "Price" : 200,
              "ShippingPrice" : 20,
              "Quantity" : 1
          }, 
          {
              "ProductId" : "504",
              "Price" : 200,
              "ShippingPrice" : 20,
              "Quantity" : 1
          }
      ]
  }
  </code></pre>
  
  <p>I'm trying to receive document with new array of <code>CartItems</code> with only <code>ProductId</code> so the response will look like:</p>
  
  <pre><code>{
      "_id" : null,
      "Status" : 0,
      "AccountId" : null,
      "Timestamp" : ISODate("2016-04-27T13:46:01.888Z"), (**default)
      "CartItems" : [ 
          {
              "ProductId" : "222",
              "Price" : 0,
              "ShippingPrice" : 0,
              "Quantity" : 0
          }, 
          {
              "ProductId" : "504",
              "Price" : 0,
              "ShippingPrice" : 0,
              "Quantity" : 0
          }, 
          {
              "ProductId" : "504",
              "Price" : 0,
              "ShippingPrice" : 0,
              "Quantity" : 0
          }, 
          {
              "ProductId" : "504",
              "Price" : 0,
              "ShippingPrice" : 0,
              "Quantity" : 0
          }
      ]
  }
  </code></pre>
  
  <p>The projection I tried (using C#) was </p>
  
  <pre><code>ProjectionDefinition&lt;Cart, Cart&gt; projectionDefinition = Builders&lt;Cart&gt;.Projection.Include(doc =&gt; doc.CartItems[0].ProductId)                                                                                      .Exclude(doc =&gt; doc.Id);
  </code></pre>
  
  <p>But the result is <code>CartItems</code> array with all default values (include <code>ProductId</code>). What I do wrong?</p>