title: Mongoose - finding subdocuments by criteria
body: <p>I've just got stuck with this problem. I've got two Mongoose schemas:</p>
  
  <pre><code>var childrenSchema = mongoose.Schema({
      name: {
          type: String
      },
      age: {
          type: Number,
          min: 0
      }
  });
  
  var parentSchema = mongoose.Schema({
      name : {
          type: String
      },
      children: [childrenSchema]
  });
  </code></pre>
  
  <p>Question is, how to fetch all subdocuments (in this case, <code>childrenSchema</code> objects) from every parent document? Let's suppose I have some data:</p>
  
  <pre><code>var parents = [
      { name: "John Smith",
      children: [
          { name: "Peter", age: 2 }, { name: "Margaret", age: 20 }
      ]},
      { name: "Another Smith",
      children: [
          { name: "Martha", age: 10 }, { name: "John", age: 22 }
      ]}
  ];
  </code></pre>
  
  <p>I would like to retrieve - in a single query - all children older than 18. Is it possible? Every answer will be appreciated, thanks!</p>