Conditionally populating a path in a Mongo query - mongodb

Question

I have the following schema:
const connectionSchema = new Schema( {
...
event: {
type: Schema.ObjectId,
ref: 'events'
},
place: {
type: Schema.ObjectId,
ref: 'places'
},
} )
const eventsSchema = new Schema( {
...
place: {
type: Schema.ObjectId,
ref: 'places'
},
} )
When I query for either a single connection or a collection of connections, I want it to check if event is specified and if so return the place from the event, otherwise return the place from the connection.
I'm currently doing something like this when I'm querying a single connection:
let c
const execFunction = ( err, connection ) => {
if ( connection.event ) {
connection.place = connection.event.place
}
c = connection
}
const connectionPromise = Connection.findById( connectionId )
connectionPromise.populate( { path: 'event', populate: { path: 'place' } } )
connectionPromise.populate( 'place' )
connectionPromise.exec( execFunction )
But, I'm trying to avoid having to iterate through the entire collection in the execFunction to do the replace logic on each individual result if I query for a collection instead.
Is there a better (i.e. more performant) way to approach this?

Answer 1

As far as I know, there is no way to do conditional population with mongoose.
In your case, I think you are trying to do "deep population".
Deep population is not supported out of the box, but there is a plugin for that! mongoose-deep-populate
const connectionQuery = Connection.findById( connectionId )
connectionQuery.populate( 'place event event.place' )
connectionQuery.exec( execFunction );
I'm guessing that 'place event.place' will probably do it: 'event' should be implied.
I also expect that, if place or event or event.place are undefined, there won't be an error, they simply won't be populated.
But I don't think there is any way, in one line, to skip populating place when event.place is available.
By the way, if you want to use promises, you could write like this:
return Connection.findById( connectionId )
.populate( 'place event event.place' )
.then( onSuccessFunc );
to propagate errors, or:
Connection.findById( connectionId )
.populate( 'place event event.place' )
.then( onSuccessFunc )
.catch( onErrorFunc );
to handle errors locally. I find that preferable to callbacks, because we don't have to worry about if (err) at every stage.