MongooseJS - population path of subdocuments

I'm having real problems with population on the below schemas. It may not be the best design of models (relatively new to MEAN stacks) but I can get it to populate everything except the Spec model.

// Spec Model
var SpecSchema = new Schema({
  time: { 
    type: Date,
    default: Date.now
  },
  active: {
    type: Boolean,
    default: 'true'
  },
  name: String,
  desc: String
});
module.exports = mongoose.model('Spec', SpecSchema);

// Thing model
var specsSchema = new Schema({
  time: { 
    type: Date,
    default: Date.now
  },
  spec: { 
    type: Schema.Types.ObjectId, 
    ref: 'Spec' 
  },
  value: String,
});
var ThingSchema = new Schema({
  time: { 
    type: Date,
    default: Date.now
  },
  active: { 
    type: Boolean,
    default: true
  },
  title: String,
  specs: [specsSchema]
});
var Thing = mongoose.model('Thing', ThingSchema);

// Set model
var thingsSchema = new Schema({
  time: { 
    type: Date,
    default: Date.now
  },
  active: {
    type: Boolean,
    default: 'true'
  },
  thing: { 
    type: Schema.Types.ObjectId, 
    ref: 'Thing' 
  }
});
var SetSchema = new Schema({
  user: { 
    type: Schema.Types.ObjectId, 
    ref: 'User' 
  },
  time: { 
    type: Date,
    default: Date.now
  },
  active: {
    type: Boolean,
    default: 'true'
  },
  title: String,
  things: [thingsSchema]
});
var Set = mongoose.model('Set', SetSchema);

The standard population is fine but i cant for the life of me get the model.populate to work and from all the examples and solutions I have looked at I'm unclear as to what the path should be.

Set.findById(req.params.id)
  .populate('things.thing')
  .populate('user', '_id name')
  .exec(function (err, set) {
    if(err) { return handleError(res, err); }
    if(!set) { return res.send(404); }
      Thing.populate(set,{
          path:'things.thing.specs.spec',
          select: 'name',
          model: Spec
      }, function(err, set){
        if ( err ) return res.json(400, err);
    });
    return res.json(set);
});

any pointers in the right direction would be much appreciated.