We found out that when projection includes "a" and "a.b", that is, parent and child document are both in the projection, the order in BSON Document is prominent.  For example:

These two are equivalent:

db.document.find({}, {"a.b":1, "a":1})
db.document.find({}, {"a":1})

and these two:

db.document.find({}, {"a":1, "a.b":1})
db.document.find({}, {"b":1})

I am wondering whether it is undocumented behavior, or it should be able to include the parent document regardless of the order.