title: What is a Mongoose model property that contains 'ref' but does not specify type?
body: <p>Very new to Mongoose -- I'm working on an existing project and have been given the task of changing some model properties.  I understand that if a model contains a property of this type</p>
  
  <pre><code>postedBy: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User'
  }
  </code></pre>
  
  <p>this property references another model/schema, and to get access to that linked model/schema one needs to <code>populate</code> it to gain access to this property.</p>
  
  <p>But in the code I'm reviewing (which I didn't write) there are many properties of this type</p>
  
  <pre><code>contentTypes: [{ ref: 'ContentType' }],
  source: { ref: 'Source',required: true },
  </code></pre>
  
  <p>where another schema is referenced, but there is no type.  Is this the same sort of relationship, and the <code>id</code> is implied? Is this a subdocument?</p>
  
  <p>As an additional question: if in a model I wanted to refer to a property of a linked model (or schema), would I need to <code>populate</code> first?   That is, if it's a subdocument, I can just use dot notation, but if it is a "linked" document, I'm not sure.</p>