MongoDB $replaceRoot Stage

db.people.drop()
db.students.drop()
db.contacts.drop()

db.people.insertMany([
  { _id: 1, name: "Arlene", age: 34, pets: { dogs: 2, cats: 1 } },
  { _id: 2, name: "Sam",    age: 41, pets: { cats: 1, fish: 3 } },
  { _id: 3, name: "Maria",  age: 25 }
])

db.students.insertMany([
  {
    _id: 1,
    studentName: "Alex",
    grades: [
      { test: 1, score: 80, subject: "Math" },
      { test: 2, score: 92, subject: "Math" },
      { test: 3, score: 88, subject: "Science" }
    ]
  },
  {
    _id: 2,
    studentName: "Jordan",
    grades: [
      { test: 1, score: 75, subject: "Math" },
      { test: 2, score: 95, subject: "Science" }
    ]
  }
])

db.contacts.insertMany([
  { _id: 1, first_name: "Gary",  last_name: "Sheffield", city: "New York" },
  { _id: 2, first_name: "Nancy", last_name: "Walker",    city: "Anaheim"  },
  { _id: 3, first_name: "Peter", last_name: "Sumner",    city: "Toledo"   }
])

db.people.aggregate([
  {
    $replaceRoot: {
      newRoot: {
        $mergeObjects: [
          { dogs: 0, cats: 0, birds: 0, fish: 0 },
          { $ifNull: ["$pets", {}] }
        ]
      }
    }
  }
])

// Maria (no pets) → { dogs: 0, cats: 0, birds: 0, fish: 0 }
// Arlene → { dogs: 2, cats: 1, birds: 0, fish: 0 }
// Original name, age, _id removed — only pet counts remain
// $ifNull prevents error when pets is missing

db.students.aggregate([
  { $unwind: "$grades" },
  { $match: { "grades.score": { $gte: 90 } } },
  {
    $replaceRoot: {
      newRoot: {
        $mergeObjects: [
          "$grades",
          { studentName: "$studentName" }
        ]
      }
    }
  },
  { $sort: { score: -1 } }
])

// Each grade object becomes the root document
// studentName merged in from parent (would be lost with bare "$grades")
// Only scores ≥ 90 kept by $match

db.contacts.aggregate([
  {
    $replaceRoot: {
      newRoot: {
        _id: "$_id",
        full_name: {
          $concat: ["$first_name", " ", "$last_name"]
        },
        location: "$city"
      }
    }
  },
  { $sort: { full_name: 1 } }
])

// Output is ONLY the three fields in newRoot
// first_name and last_name consumed by $concat — not in output
// Clean API-friendly shape from messy source columns

db.contacts.aggregate([
  {
    $replaceRoot: {
      newRoot: {
        $mergeObjects: [
          { _id: "", first_name: "", last_name: "", city: "", phone: "" },
          "$$ROOT"
        ]
      }
    }
  },
  {
    $project: {
      _id: 1,
      full_name: { $concat: ["$first_name", " ", "$last_name"] },
      city: 1,
      phone: 1
    }
  }
])

// Default empty strings for missing phone on all contacts
// $$ROOT = entire current document inside newRoot expression
// Then $project for final API shape