title: mongoose populate reference to subdocument
body: <p>I could not find any response to my issue. Probably also a search issue, but would appreciate some help. 
  I have the following model: </p>
  
  <pre><code>var ChildSchema =  new Schema ({
      'name': string
  });
  var ClassSchema =  new Schema ({
      'name': string, 
      'children': [ChildSchema] 
  });
  </code></pre>
  
  <p>I have only declared a model for class. </p>
  
  <pre><code>mongoose.model ('Class', ClassSchema); 
  </code></pre>
  
  <p>and then I have another model: </p>
  
  <pre><code>var TeacherSchema = new Schema ({
     'name': string, 
     'favorite': {type: ObjectId, ref: 'Class.children'} // a child from class children
     'least': {type: ObjectId, ref: 'Class.children'} // same as above
  }); 
  </code></pre>
  
  <p>With relevant model: </p>
  
  <pre><code>mongoose.model ('Teacher', TeacherSchema); 
  </code></pre>
  
  <p>I am trying to retrieve a teacher by ID and get the children name populated: </p>
  
  <pre><code>Teacher.findbyId (teacherId).populate (favorite least)... 
  </code></pre>
  
  <p>This does not seem to work. Is this modelling correct? Is this possible? </p>
  
  <p>(I am aware that my example might not be politically correct, so please forgive me...) </p>
  
  <p><strong>EDIT</strong>
  This is a reflection of something in project. I am aware that the children can be modelled as seperate collection, but in my project, I prefer to keep them as embedded documents (for various reasons). 
  Each child does get an _id when pushed into  the children array and class is saved. The teacher needs to point to a single child in a class in "favorite" and in "least", i.e. to keep the _id of that child.  </p>