Populate a nested ObjectRef - coffeescript

Question

So here's my model:
CompetitionSchema = new Mongoose.Schema
name :
type : String
required : true
required_teams :
type : Number
teams : [
_team :
type : Mongoose.Schema.ObjectId
ref : 'teams'
weight :
type : Number
min : 0
]
I would like to be able to populate a competition object in order to access competition.teams[0]._team._id for which I have tried the following:
models('Competition')
.findById id
.populate('teams._team')
.exec (error, competition) ->
if error || !competition
error_callback error
else
success_callback competition
However this has no effect. I've also tried:
models('Competition')
.findById id
.exec (error, competition) ->
if error || !competition
error_callback error
else
options = [
path : 'teams._team'
model : 'teams'
]
models('Competition')
.populate(
competition
, options
, (error, competition) ->
if error || !competition
error_callback competition
else
success_callback competition
)
Also to no effect. I find the API documentation for Model.populate to be quite confusing, so please excuse me if it's plainly obvious!

Answer 1

OK after soaking myself in the topic a bit more I finally came to the below:
models('Competition')
.findById id
.exec (error, competition) ->
if error || !competition
error_callback error
else
options = [
path : '_team'
model : 'teams'
]
models('Team')
.populate(
competition.teams
, '_team'
, (error, competition_teams) ->
if error || !competition_teams
error_callback competition
else
success_callback competition
)
It boils down to: Instead of calling
Competition.populate(competition, { path : 'teams._team' } , callback);
I am now calling:
Team.populate(competition.teams, '_team', callback);
This has the benefit of modifying the competition.teams array rather than returning a copy, so the original competition object receives the changes too!