Add a $transform aggregation stage

$transform: {

	root: <expression evaluates to object>,

	set: {<fieldname>: <arbitrary expression>}

}

// =====  Example #1 - Promoting a subfield to the top level =====

>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})

>db.example.aggregate([{$transform: {root: “$a1”}]);

{b: 1}

// =====  Example #2 - Creating a new field =====

>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})

>db.example.aggregate([{$transform: {set: {a3: 3}}}])

{_id: 0, a1: {b: 1}, a2: 2, a3: 3}

// =====  Example #3 - Creating new calculated fields =====

>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})

>db.example.aggregate([{$transform: {set: {a3: {$gt: ["$a2", 3]}, a4: 4 }}}])

{_id: 0, a1: {b: 1}, a2: 2, a3: false, a4: 4}

// =====  Example #4 - Promoting a subfield to the top level and then creating new fields =====

>db.example.insert({_id: 0, a1: {b: 1}, a2: 2 })

>db.example.aggregate([{$transform: {root: "$a1", set: {c: 3}}}])

{b: 1, c: 3}

// =====  Example #5 - Error: using value of a field that doesn't exist anymore =====

>db.example.insert({_id: 0, a1: {b: 1}, a2: 2 })

>db.example.aggregate([{$transform: {root: "$a1", set: {c: "$a2"}}}])

Error

// Allowing this is a non-goal of this ticket.