{$mergeObjects: [<expression 1>, <expression 2>, ..., <expression N>]}

// 1. Merging two objects together
db.merging.insert({_id: 0, subObject: {b: 1, c: 1}})
db.merging.aggregate([{$project: {newDocument: {$mergeObjects: ["$subObject", {d: 1}]}}}]);
// Results:
{_id: 0, newDocument: {b: 1, c: 1, d: 1}}

// 2. Merging the root with a new field
db.merging.insert({_id: 0, field0: 0, field1: 1})
db.merging.aggregate([{$project: {newDocument: {$mergeObjects: ["$$ROOT", {newField: "newValue"}]}}}]);
// Results:
{_id: 0, newDocument: {_id: 0, field0: 0, field1: 1, newField: "newValue"}}

// 3. Replacing a field in the root.
db.merging.insert({_id: 0, field0: 0, field1: 1})
db.merging.aggregate([{$project: {newDocument: {$mergeObjects: ["$$ROOT", {field0: "newValue"}]}}}]);
// Results:
{_id: 0, newDocument: {_id: 0, field0: "newValue", field1: 1}}

// 4. Overriding in the opposite order
db.merging.insert({_id: 0, field0: 0, field1: 1})
db.merging.aggregate([{$project: {newDocument: {$mergeObjects: [{field0: "defaultValue"}, "$$ROOT"]}}}]);
// Results:
{_id: 0, newDocument: {_id: 0, field0: 0, field1: 1}}

db.merging.insert({_id: 0, field0: 0, subdoc: {a: 1, b: 3}})
db.merging.aggregate([{$replaceRoot: {newRoot: {$mergeObjects: ["$$ROOT", "$subdoc"]}}}])
// results:
{_id: 0, field0: 0, subdoc: {a: 1, b: 3}, a: 1, b: 3}