You can use MongoDB projection i.e $project
aggregation framework pipeline operators as well. (recommended way). If you don’t want to use project
check this link
db.collection.aggregation([{$project{ . . }}]);
Below is the example for your case:
db.collectionName.aggregate
([
{
$project: {
a: 1,
b: '$subdoc.b',
c: '$subdoc.c'
}
}
]);
Gives you the output as you expected i.e.
{
"a" : 1,
"b" : 2,
"c" : 3
}