Add builder for $addFields aggregation stage

{$addFields: {newFieldName1: <Expression1>, ...}}

// =====  Example #1 - Adding a literal field value =====

>db.example.insert({_id: 0, a: 1})

>db.example.aggregate([{$addFields: {newField: “hello”}}]);

{_id: 0, a: 1, newField: “hello”}

// =====  Example #2 - Adding a value from a field path =====

>db.example.insert({_id: 0, a: 1})

>db.example.aggregate([{$addFields: {b: “$a”}}])

{_id: 0, a: 1, b: 1}

// =====  Example #3 - Adding a value from a system variable =====

>db.example.insert({_id: 0, a: 1})

>db.example.aggregate([{$addFields: {this: “$$CURRENT”}}])

{_id: 0, a: 1, this: {_id: 0, a: 1}}

// =====  Example #4 - Adding a value from an expression object =====

>db.example.insert({_id: 0, a: 1})

>db.example.aggregate([{$addFields: {myNewField: {c: 3, d: 4}}}])

{_id: 0, a: 1, myNewField: {c: 3, d: 4}}

// =====  Example #5 - Adding a value from an operator expression =====

>db.example.insert({_id: 0, a: 1})

>db.example.aggregate([{$addFields: {alt3: {$lt: [“$a”, 3]}}}])

{_id: 0, a: 1, alt3: true}

// =====  Example #6 Adding multiple new fields at once =====

>db.example.insert({_id: 0, a: 1})

>db.example.aggregate([{$addFields: {b: 3, c: 5}}])

{_id: 0, a: 1, b: 3, c: 5}

// =====  Example #6 - Setting a field that already exists =====

>db.example.insert({_id: 0, a: 1})

>db.example.aggregate([{$addFields: {a: [1, 2, 3]}}])

{_id: 0, a: [1, 2, 3]}