Or another way of stating the issue is, I cannot change array field to non-array field if I want to use any of its contents in the expression (without changing its name).

db.coll.insert({a:[1,2]})

I can change it to be embedded inside another field like this:

db.coll.aggregate({$project:{b:{a:"$a"},_id:0}})
{ "b" : { "a" : [ 1, 2 ] } }

However, if I need the top level name of new field is to be "a" then there seems to be no way to avoid projection happening for every element of original array:

// I want to get back {"a" : {"b" : [1, 2]} }
db.coll.aggregate({$project:{a:{b:"$a"},_id:0}})
{ "a" : [ { "b" : [ 1, 2 ] }, { "b" : [ 1, 2 ] } ] }

I haven't found a way to do this except by having two $projects, one to create newA and the second to rename newA to a. I understanding adding fields to existing arrays of objects requires preserving cardinality (and type) of array field, but this seems to be a full replace.

Looked for existing ticket related to this, SERVER-25200 is possibly the closest (though it mentions dotted notation, the behavior is similar).