title: How to make projections of selected attributes in mongoid?
body: <p>I have Coordinate model with many markers embedded_in User model. How to extract attributes without _ids so that on output display only each lng and lat?</p>
  
  <p>structure:</p>
  
  <pre><code>{ "_id" : ObjectId( "4e9f418f1e7bf20fbc000009" ),
    "name" : "test",
    "coordinates" : [ 
      { "lng" : 16.86783310009764,
        "lat" : 52.38353842845282,
        "_id" : ObjectId( "4e9f418f1e7bf20fbc00000a" ) }, 
      { "lng" : 16.85787674023436,
        "lat" : 52.40972501601293,
        "_id" : ObjectId( "4e9f418f1e7bf20fbc00000b" ) }, 
      { "lng" : 16.92276474072264,
        "lat" : 52.40071858320756,
        "_id" : ObjectId( "4e9f418f1e7bf20fbc00000c" ) }, 
      { "lng" : 16.90182205273436,
        "lat" : 52.38270020105396,
        "_id" : ObjectId( "4e9f418f1e7bf20fbc00000d" ) }, 
      { "lng" : 16.96705337597655,
        "lat" : 52.410661698108,
        "_id" : ObjectId( "4e9f418f1e7bf20fbc00000e" ) }, 
      { "lng" : 16.89495559765624,
        "lat" : 52.42773236584494,
        "_id" : ObjectId( "4e9f418f1e7bf20fbc00000f" ) } ] }
  </code></pre>
  
  <p>e.g.</p>
  
  <pre><code>= debug @user.coordinates.to_json
  </code></pre>
  
  <p>gives:</p>
  
  <pre><code>--- ! '[{"_id":"4e9f418f1e7bf20fbc00000a","lat":52.383538428452816,"lng":16.86783310009764},{"_id":"4e9f418f1e7bf20fbc00000b","lat":52.40972501601293,"lng":16.85787674023436},{"_id":"4e9f418f1e7bf20fbc00000c","lat":52.40071858320756,"lng":16.92276474072264},{"_id":"4e9f418f1e7bf20fbc00000d","lat":52.382700201053964,"lng":16.90182205273436},{"_id":"4e9f418f1e7bf20fbc00000e","lat":52.410661698108,"lng":16.967053375976548},{"_id":"4e9f418f1e7bf20fbc00000f","lat":52.42773236584494,"lng":16.894955597656235}]'
  </code></pre>
  
  <p>expected:</p>
  
  <pre><code>--- ! '[{"lat":52.383538428452816,"lng":16.86783310009764},{"lat":52.40972501601293,"lng":16.85787674023436},{"lat":52.40071858320756,"lng":16.92276474072264},{"lat":52.382700201053964,"lng":16.90182205273436},{"lat":52.410661698108,"lng":16.967053375976548},{"lat":52.42773236584494,"lng":16.894955597656235}]'
  </code></pre>