MongoDB $group will not allow me to $project extra fields - mongodb

Question

I almost got this one working, but I simply cannot figure out why the $project part does not work for normal fields....
This is "invoice" table:
{
"_id" : "AS6D0",
"invoiceNumber" : 23,
"bookingId" : "AS6D0",
"createDate" : 1490369414,
"dueDate" : 1490369414,
"invoiceLines" : [
{
"lineText" : "Rent Price",
"amountPcs" : "8 x 7500",
"amountTotal" : 60000
},
{
"lineText" : "Discount(TIKO10)",
"amountPcs" : "10%",
"amountTotal" : -10000
},
{
"lineText" : "Final cleaning",
"amountPcs" : "1 x 5000",
"amountTotal" : 5000
},
{
"lineText" : "Reservation fee paid already",
"amountPcs" : "1 x -20000",
"amountTotal" : -20000
}
],
"managerId" : "4M4KE"
}
And this is my query
db.getCollection('invoice').aggregate([
{
$match: {
bookingId: "AS6D0"
}
},
{
$unwind: "$invoiceLines"
},
{
$group: {
_id: "$_id",
sum: {$sum: "$invoiceLines.amountTotal"}
}
},
{
$project:{
"_id" : 0,
"invoiceNumber" : 1,
"dueDate" : 1,
"sum" : 1
}
}
])
I get the _id and the sum, but it wont show invoiceNumber and dueDate

Answer 1

You could use a trick like this :
db.getCollection('invoice').aggregate([
{ $match: { } },
{ $unwind: "$invoiceLines" },
{ $group: { _id: "$_id",
sum: {$sum: "$invoiceLines.amountTotal"},
invoiceNumber: { $addToSet: "$invoiceNumber" },
dueDate: { $addToSet: "$dueDate" } } }
]);

Answer 2

Thanks to Mateo, this is what I ended up with:
(I do the unwind on the fields to avoid single value arrays)
Update : You don't have to $addToSet to reduce the fields into single value arrays and $unwind. Use $first instead.
db.getCollection('invoice').aggregate([
{
$match: {
bookingId: "AS6D0"
}
},
{
$unwind: "$invoiceLines"
},
{
$group: {
_id: "$_id",
sum: {$sum: "$invoiceLines.amountTotal"},
invoiceNumber: { $first: "$invoiceNumber" },
dueDate: { $first: "$dueDate" }
}
},
{
$project:{
"_id" : 0,
"invoiceNumber" : 1,
"dueDate" : 1,
"sum" : 1
}
}
])