MongoDB (Mgo v2) Projection returns parent struct - mongodb

Question

I have here a Building Object where inside sits an Array of Floor Objects.
When Projecting, my goal is to return or count the number of Floor Objects inside a Building Object after matching the elements accordingly. The code is as follows:
Objects:
type Floor struct {
// Binary JSON Identity
ID bson.ObjectId `bson:"_id,omitempty"`
// App-level Identity
FloorUUID string `bson:"f"`
// Floor Info
FloorNumber int `bson:"l"`
// Units
FloorUnits []string `bson:"u"`
// Statistics
Created time.Time `bson:"y"`
}
type Building struct {
// Binary JSON Identity
ID bson.ObjectId `bson:"_id,omitempty"`
// App-level Identity
BldgUUID string `bson:"b"`
// Address Info
BldgNumber string `bson:"i"` // Street Number
BldgStreet string `bson:"s"` // Street
BldgCity string `bson:"c"` // City
BldgState string `bson:"t"` // State
BldgCountry string `bson:"x"` // Country
// Building Info
BldgName string `bson:"w"`
BldgOwner string `bson:"o"`
BldgMaxTenant int `bson:"m"`
BldgNumTenant int `bson:"n"`
// Floors
BldgFloors []Floor `bson:"p"`
// Statistics
Created time.Time `bson:"z"`
}
Code:
func InsertFloor(database *mgo.Database, bldg_uuid string, fnum int) error {
fmt.Println(bldg_uuid)
fmt.Println(fnum) // Floor Number
var result Floor // result := Floor{}
database.C("buildings").Find(bson.M{"b": bldg_uuid}).Select(
bson.M{"p": bson.M{"$elemMatch": bson.M{"l": fnum}}}).One(&result)
fmt.Printf("AHA %s", result)
return errors.New("x")
}
It turns out, no matter how I try the query returns a Building Object, not a floor object? What changes do I need to make in order to have the query fetch and count Floors and not Buildings?
This is done so to check if a Floor inside Building already exists before insertion. If there's a better a approach then I'll replace mine with the better!
Thanks!

Answer 1

You are querying for a Building document so mongo returns that to you even though you try to mask some of its fields using projection.
I don't know of a way to count the number of elements in a mongo array in a find query, but you can use the aggregation framework, where you have the $size operator that does exactly this. So you should send a query like this to mongo :
db.buildings.aggregate([
{
"$match":
{
"_id": buildingID,
"p": {
"$elemMatch": {"l": fNum}
}
}
},
{
"$project":
{
nrOfFloors: {
"$size": "$p"
}
}
}])
Which in go it would look like
result := []bson.M{}
match := bson.M{"$match": bson.M{"b": bldg_uuid, "p": bson.M{"$elemMatch": bson.M{"l": fNum}}}}
count := bson.M{"$project": bson.M{"nrOfFloors": bson.M{"$size": "$p"}}}
operations := []bson.M{match, count}
pipe := sess.DB("mgodb").C("building").Pipe(operations)
pipe.All(&result)