title: Add to an object to Population in a Mongoose Model
body: <p>I am new to Mongoose and getting my head around populations. I have Schema that looks like...</p>
  
  <pre><code>var ProjectSchema = new Schema({
      name: {
          type: String,
          default: '',
          required: 'Please fill Project name',
          trim: true
      },
      created: {
          type: Date,
          default: Date.now
      },
      topics: [{
          type: Schema.ObjectId,
          ref: 'Topic'
      }]
  });
  </code></pre>
  
  <p>When I am querying for projects I use .populate('topics') and it works fine. When the project is populated the topics property holds actual objects rather than references.</p>
  
  <p>P.S. The Topic does not have a project reference (I found it difficult to maintain resiprocal relations in MongoDB).</p>
  
  <p>My question is: <strong>when I want to add a topic object to a project. Do I add the ObjectId or the object itself?</strong> </p>