mongodb: how to populate a single nested subdocuments

Question

I am writing a node.js application using mongodb and mongoose. I seems to be having an issue in understanding how to query "single nested subdocuments". If I have an example like the following:

var childSchema = new Schema({ name: 'string' });

var parentSchema = new Schema({
  // Array of subdocuments
  children: [childSchema],
  // Single nested subdocuments. Caveat: single nested subdocs only work
  // in mongoose >= 4.2.0
  child: childSchema
});

from the mongoose documentation, how do I populate the child subdocument for a single nested scenario? I tried to use the ".populate("child").exec..." but the child object returns null even though I can find if I run db.child.find() from the mongo command line. Now I saw in the documentation, where you have to call "parent.children.id(_id)" but in this case I will not know the _id ahead of time.

Update: I removed the populate and it appears to be working now.

Answer 1

You are already storing the child document as embedded one inside the parent document, so there is no need to populate. Populate is for those cases where you are storing a reference to other document which you want to be populated while running the query. An example for such case would be :

var childSchema = new Schema({ name: 'string' });
var childModel = mongoose.model("child", childSchema);

var parentSchema = new Schema({
  // Array of subdocuments
  children: [childSchema],
  child: {
    type: Schema.ObjectId, ref: 'child'
  }
});

Now if you try to populate child while querying parent , it should work.