Inserting Documents

Inserting data is the first operation you will perform in MongoDB. The database provides two methods for inserting documents: insertOne for a single document and insertMany for multiple documents at once.

insertOne

insertOne inserts a single document into a collection. If the collection does not exist, MongoDB creates it automatically.

db.users.insertOne({
  name: "Alice",
  email: "alice@example.com",
  age: 28,
  createdAt: new Date()
})

The method returns an object with an acknowledged flag and the insertedId of the new document.

{
  acknowledged: true,
  insertedId: ObjectId("64c3d4e5f6a7b8c9d0e1f002")
}

insertMany

insertMany accepts an array of documents and inserts them all in a single round trip, which is far more efficient than calling insertOne in a loop.

db.products.insertMany([
  { name: "Keyboard", price: 49.99, stock: 150 },
  { name: "Mouse",    price: 29.99, stock: 200 },
  { name: "Monitor",  price: 299.00, stock: 40 }
])

The result includes an array of all insertedIds.

The _id Field

Every document must have a unique _id field. If you omit it, MongoDB generates an ObjectId automatically. You can also supply your own _id — any unique value works, including strings or integers:

db.countries.insertOne({ _id: "GB", name: "United Kingdom" })

Ordered vs Unordered Inserts

By default, insertMany is ordered: if one document fails (for example, due to a duplicate _id), MongoDB stops processing the remaining documents. Pass { ordered: false } to continue inserting the rest even if some fail:

db.logs.insertMany(
  [
    { _id: 1, msg: "start" },
    { _id: 1, msg: "duplicate" },  // will fail
    { _id: 2, msg: "end" }
  ],
  { ordered: false }
)

Write Concern

Write concern controls how many acknowledgements the driver waits for before confirming a write. The default { w: 1 } waits for the primary to acknowledge. For maximum durability you can use { w: "majority" } to wait for a majority of replica set members.

db.orders.insertOne(
  { item: "Widget", qty: 5 },
  { writeConcern: { w: "majority" } }
)

Inserting Nested Documents and Arrays

MongoDB handles complex structures natively:

db.articles.insertOne({
  title: "NoSQL Explained",
  author: { name: "Bob", email: "bob@example.com" },
  tags: ["database", "nosql"],
  sections: [
    { heading: "Introduction", wordCount: 320 },
    { heading: "Examples",     wordCount: 640 }
  ]
})