{$replaceRoot: {newRoot: <expression that evaluates to a document>}}

// =====  Example #1 - Promoting a subfield to the top level =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
>db.example.aggregate([{$replaceRoot: {newRoot: “$a1”}}]);
{b: 1}

// =====  Example #2 - Promoting a nested field to the top level =====
>db.example.insert({_id: 0, a1: {b: {c1: 4, c2: 5}}, a2: 2})
>db.example.aggregate([{$replaceRoot: {newRoot: “$a1.b”}}])
{_id: 0, c1: 4, c2: 5}

// =====  Example #3 - Promoting a field that contains an _id =====
>db.example.insert({_id: 0, a1: {b: 1, _id: 7}, a2: 2})
>db.example.aggregate([{$replaceRoot: {newRoot: “$a1”}}]);
{b: 1, _id: 7}

// =====  Errors #1 - Invalid path syntax =====
>db.example.aggregate([{$replaceRoot: “$a” }])
Error: expected an object as specification for $replaceRoot stage

// =====  Error #2 - When the field doesn’t exist =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2})
>db.example.aggregate([{$replaceRoot: {newRoot: “$c”}}])
Error: newRoot must resolve to an object

// =====  Error #3 - When the field is not an object =====
>db.example.insert({_id: 0, a1: {b: 1}, a2: 2 })
>db.example.aggregate([{$replaceRoot: {newRoot: “$a2”}}])
Error: newRoot must resolve to an object

{
  _id: "user_id",
  first: "first name",
  last: "last name",
  addresses: [
    {city: "New York", state: "NY", zip: "10036", address: "229 W 43rd"},
    {city: "Palo Alto", state: "CA", zip: "94301", address: "100 Forest Ave"}
    ...
  ]
}

db.users.aggregate([
  {$unwind: "$addresses"},
  {$transform: {root: "$addresses"}}
])

{city: "New York", state: "NY", zip: "10036", address: "229 W 43rd"}
{city: "Palo Alto", state: "CA", zip: "94301", address: "100 Forest Ave"}