title: Why arrays of objects has ids on Mongoose?
body: <p>I've went through some problems while saving some documents and realized that some inner documents has id references.</p>
  
  <p>Let's take this schema for example:</p>
  
  <pre><code>var VisitSchema = mongoose.Schema({
      user_id: { type: Schema.Types.ObjectId, ref: 'User' },
      name: String,
      city: {
          name: String,
          reference: String,
          location: [Number]
      },
      photos: [{
          image_url: String,
          description: String
      }]
  });
  </code></pre>
  
  <p>When this schema is saved, here is the resulting document:</p>
  
  <pre><code>{
      "_id" : ObjectId("5449eceeea5281056768aef8"),
      "user_id" : ObjectId("540603dd797be100008b4340"),
      "name" : "Test",
      "photos" : [ 
          {
              "image_url" : "testURL",
              "description" : "testDescription",
              "_id" : ObjectId("5449eceeea5281056768aefb")
          }
      ],
      "city" : {
          "name" : "New York, NY, United States",
          "reference" : "CoQBdAAAAFfbZuaWptFtAkhGaO87UVSaXbyYwtTY_xX-pH84mS6QO2ypLr15znsCnYvaZ_N8CvwCBAr4y34PSQqyTUwxa5qbUKBK0yHnUFgTLKPQMFKqyQ8xd4vcDPNXf5XSxlpsXKKPYnU0AJZEVVxXLJ_6IBiUay9emFvNXPiYb7kT04NjEhB8fK-jKIZHJ5Bfz_mfyk2MGhTJwOPU68NQiV7RlAdb0Gca1bg0ew",
          "location" : [ 
              -74.0059413, 
              40.7127837
          ]
      },
      "__v" : 0
  }
  </code></pre>
  
  <p>Why does the subdocument <code>photos</code> has an <strong>id</strong> reference? It doesn't make sense.</p>
  
  <p>Thanks.</p>