Insert Documents in MongoDB

Use db.collection.insert() (or its two modern variants below) to add new documents to a MongoDB collection. You don't need to create the collection first — inserting into a non-existent collection creates it automatically.

There are two main methods for inserting documents:

• insertOne() — insert a single document
• insertMany() — insert multiple documents at once

insertOne()

db.emp.insertOne({ "name": "Rahul", "email": "rahul@gmail.com" })
// {
//   acknowledged: true,
//   insertedId: ObjectId("6490ae32a1ada8a3a0c0ad76")
// }

Nothing stops you from inserting a second document with the same email:

db.emp.insertOne({ "name": "Rahil", "email": "rahul@gmail.com" })

Two employees now share the same email — MongoDB won't complain unless you explicitly enforce uniqueness with a unique index (see the Indexing section).

Insert an Array

Wrap values in square brackets [ ] to store a list under one field — the array can hold any mix of data types.

db.emp.insertOne({
  "name": "Roshan",
  "email": "roshan@gmail.com",
  "marks": [100, 200, 300, 400]
})

Insert an Embedded Object

Use curly braces { } with key-value pairs to nest a document inside another document.

db.emp.insertOne({
  "name": "Rahit",
  "email": "rohit@gmail.com",
  "reporting": {
    "managerid": 121,
    "managername": "shubham",
    "managerdesignation": "Sr Software Engineer"
  }
})

insertMany()

Pass an array of documents to insert several at once in a single call.

db.emp.insertMany([
  { "name": "mangesh", "email": "mangesh@gmail.com" },
  { "name": "hitesh", "email": "hitesh@gmail.com" },
  { "name": "harshit", "email": "harshit@gmail.com" }
])
// {
//   acknowledged: true,
//   insertedIds: { '0': ObjectId(...), '1': ObjectId(...), '2': ObjectId(...) }
// }

Enforcing Uniqueness with createIndex()

If duplicate values (like the email above) are a problem, create a unique index on that field:

db.emp.createIndex({ email: 1 }, { unique: true })

If duplicates already exist, MongoDB refuses to build the index until you remove them:

E11000 duplicate key error collection: rahultechnology.emp index: email_1
dup key: { email: "rahul@gmail.com" }

Once the duplicates are gone and the index is created, any future insert with a repeated email fails immediately:

db.emp.insertOne({ "name": "Rahil", "email": "rahul@gmail.com" })
// MongoServerError: E11000 duplicate key error ... dup key: { email: "rahul@gmail.com" }

Common Errors & Troubleshooting

• E11000 duplicate key error — you tried to insert a value that violates a unique index (like a repeated email once createIndex({email:1},{unique:true}) is in place). Check the existing documents for the conflicting value, or use updateOne(..., {upsert:true}) instead if you meant to update.
• insertMany() stops partway through on an error — by default, insertMany() halts on the first failed document (like a duplicate key) and doesn't insert the rest. Pass { ordered: false } if you want it to skip failures and keep inserting the remaining documents.
• Forgetting quotes around field names with special characters — a field name containing a dot or dollar sign (like "price.usd") needs to be quoted; MongoDB reserves unquoted dots for addressing nested fields.

Key Takeaways

• insertOne() inserts a single document; insertMany() inserts an array of them.
• Arrays use [ ], embedded documents use { } — both can be inserted directly.
• MongoDB allows duplicate field values by default — use createIndex({ field: 1 }, { unique: true }) to prevent them.