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Zhongwei Li
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skills/databases/references/mongodb-crud.md
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408
skills/databases/references/mongodb-crud.md
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# MongoDB CRUD Operations
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CRUD operations (Create, Read, Update, Delete) in MongoDB with query operators and atomic updates.
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## Create Operations
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### insertOne
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```javascript
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// Insert single document
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db.users.insertOne({
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name: "Alice",
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email: "alice@example.com",
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age: 30,
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createdAt: new Date()
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})
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// Returns: { acknowledged: true, insertedId: ObjectId("...") }
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```
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### insertMany
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```javascript
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// Insert multiple documents
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db.users.insertMany([
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{ name: "Bob", age: 25 },
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{ name: "Charlie", age: 35 },
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{ name: "Diana", age: 28 }
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])
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// With ordered: false (continue on error)
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db.users.insertMany(docs, { ordered: false })
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```
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## Read Operations
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### find
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```javascript
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// Find all documents
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db.users.find()
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// Find with filter
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db.users.find({ age: { $gte: 18 } })
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// Projection (select fields)
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db.users.find({ status: "active" }, { name: 1, email: 1, _id: 0 })
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// Cursor operations
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db.users.find()
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.sort({ createdAt: -1 })
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.limit(10)
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.skip(20)
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```
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### findOne
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```javascript
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// Get single document
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db.users.findOne({ email: "alice@example.com" })
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// With projection
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db.users.findOne({ _id: ObjectId("...") }, { name: 1, email: 1 })
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```
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### count/estimatedDocumentCount
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```javascript
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// Count matching documents
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db.users.countDocuments({ status: "active" })
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// Fast estimate (uses metadata)
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db.users.estimatedDocumentCount()
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```
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### distinct
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```javascript
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// Get unique values
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db.users.distinct("status")
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db.users.distinct("city", { country: "USA" })
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```
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## Update Operations
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### updateOne
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```javascript
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// Update first matching document
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db.users.updateOne(
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{ email: "alice@example.com" },
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{ $set: { status: "verified" } }
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)
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// Upsert (insert if not exists)
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db.users.updateOne(
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{ email: "new@example.com" },
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{ $set: { name: "New User" } },
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{ upsert: true }
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)
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```
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### updateMany
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```javascript
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// Update all matching documents
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db.users.updateMany(
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{ lastLogin: { $lt: cutoffDate } },
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{ $set: { status: "inactive" } }
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)
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// Multiple updates
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db.users.updateMany(
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{ status: "pending" },
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{
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$set: { status: "active" },
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$currentDate: { updatedAt: true }
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}
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)
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```
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### replaceOne
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```javascript
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// Replace entire document (except _id)
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db.users.replaceOne(
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{ _id: ObjectId("...") },
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{ name: "Alice", email: "alice@example.com", age: 31 }
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)
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```
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## Delete Operations
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### deleteOne
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```javascript
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// Delete first matching document
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db.users.deleteOne({ email: "alice@example.com" })
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```
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### deleteMany
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```javascript
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// Delete all matching documents
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db.users.deleteMany({ status: "deleted" })
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// Delete all documents in collection
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db.users.deleteMany({})
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```
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## Query Operators
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### Comparison Operators
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```javascript
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// $eq (equals)
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db.users.find({ age: { $eq: 30 } })
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db.users.find({ age: 30 }) // Implicit $eq
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// $ne (not equals)
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db.users.find({ status: { $ne: "deleted" } })
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// $gt, $gte, $lt, $lte
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db.users.find({ age: { $gt: 18, $lte: 65 } })
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// $in (in array)
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db.users.find({ status: { $in: ["active", "pending"] } })
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// $nin (not in array)
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db.users.find({ status: { $nin: ["deleted", "banned"] } })
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```
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### Logical Operators
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```javascript
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// $and (implicit for multiple conditions)
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db.users.find({ age: { $gte: 18 }, status: "active" })
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// $and (explicit)
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db.users.find({
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$and: [
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{ age: { $gte: 18 } },
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{ status: "active" }
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]
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})
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// $or
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db.users.find({
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$or: [
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{ status: "active" },
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{ verified: true }
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]
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})
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// $not
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db.users.find({ age: { $not: { $lt: 18 } } })
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// $nor (not any condition)
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db.users.find({
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$nor: [
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{ status: "deleted" },
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{ status: "banned" }
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]
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})
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```
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### Element Operators
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```javascript
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// $exists
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db.users.find({ phoneNumber: { $exists: true } })
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db.users.find({ deletedAt: { $exists: false } })
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// $type
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db.users.find({ age: { $type: "int" } })
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db.users.find({ age: { $type: ["int", "double"] } })
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```
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### Array Operators
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```javascript
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// $all (contains all elements)
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db.posts.find({ tags: { $all: ["mongodb", "database"] } })
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// $elemMatch (array element matches all conditions)
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db.products.find({
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reviews: {
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$elemMatch: { rating: { $gte: 4 }, verified: true }
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}
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})
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// $size (array length)
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db.posts.find({ tags: { $size: 3 } })
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```
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### String Operators
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```javascript
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// $regex (regular expression)
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db.users.find({ name: { $regex: /^A/i } })
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db.users.find({ email: { $regex: "@example\\.com$" } })
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// Text search (requires text index)
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db.articles.find({ $text: { $search: "mongodb database" } })
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```
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## Update Operators
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### Field Update Operators
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```javascript
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// $set (set field value)
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db.users.updateOne(
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{ _id: userId },
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{ $set: { status: "active", updatedAt: new Date() } }
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)
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// $unset (remove field)
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db.users.updateOne(
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{ _id: userId },
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{ $unset: { tempField: "" } }
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)
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// $rename (rename field)
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db.users.updateMany(
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{},
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{ $rename: { "oldName": "newName" } }
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)
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// $currentDate (set to current date)
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db.users.updateOne(
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{ _id: userId },
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{ $currentDate: { lastModified: true } }
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)
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```
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### Numeric Update Operators
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```javascript
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// $inc (increment)
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db.posts.updateOne(
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{ _id: postId },
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{ $inc: { views: 1, likes: 5 } }
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)
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// $mul (multiply)
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db.products.updateOne(
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{ _id: productId },
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{ $mul: { price: 1.1 } } // 10% increase
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)
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// $min (update if new value is less)
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db.scores.updateOne(
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{ _id: scoreId },
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{ $min: { lowestScore: 50 } }
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)
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// $max (update if new value is greater)
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db.scores.updateOne(
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{ _id: scoreId },
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{ $max: { highestScore: 100 } }
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)
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```
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### Array Update Operators
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```javascript
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// $push (add to array)
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db.posts.updateOne(
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{ _id: postId },
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{ $push: { comments: { author: "Alice", text: "Great!" } } }
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)
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// $push with $each (multiple elements)
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db.posts.updateOne(
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{ _id: postId },
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{ $push: { tags: { $each: ["mongodb", "database"] } } }
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)
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// $addToSet (add if not exists)
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db.users.updateOne(
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{ _id: userId },
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{ $addToSet: { interests: "coding" } }
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)
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// $pull (remove matching elements)
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db.users.updateOne(
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{ _id: userId },
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{ $pull: { tags: "deprecated" } }
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)
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// $pop (remove first/last element)
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db.users.updateOne(
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{ _id: userId },
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{ $pop: { notifications: -1 } } // -1: first, 1: last
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)
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// $ (update first matching array element)
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db.posts.updateOne(
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{ _id: postId, "comments.author": "Alice" },
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{ $set: { "comments.$.text": "Updated comment" } }
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)
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// $[] (update all array elements)
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db.posts.updateOne(
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{ _id: postId },
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{ $set: { "comments.$[].verified": true } }
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)
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// $[<identifier>] (filtered positional)
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db.posts.updateOne(
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{ _id: postId },
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{ $set: { "comments.$[elem].flagged": true } },
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{ arrayFilters: [{ "elem.rating": { $lt: 2 } }] }
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)
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```
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## Atomic Operations
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### findAndModify / findOneAndUpdate
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```javascript
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// Find and update (returns old document by default)
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db.users.findOneAndUpdate(
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{ email: "alice@example.com" },
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{ $set: { status: "active" } }
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)
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// Return new document
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db.users.findOneAndUpdate(
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{ email: "alice@example.com" },
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{ $set: { status: "active" } },
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{ returnNewDocument: true }
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)
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// Upsert and return new
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db.counters.findOneAndUpdate(
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{ _id: "sequence" },
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{ $inc: { value: 1 } },
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{ upsert: true, returnNewDocument: true }
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)
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```
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### findOneAndReplace
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```javascript
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// Find and replace entire document
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db.users.findOneAndReplace(
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{ _id: ObjectId("...") },
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{ name: "Alice", email: "alice@example.com" },
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{ returnNewDocument: true }
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)
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```
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### findOneAndDelete
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```javascript
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// Find and delete (returns deleted document)
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const deletedUser = db.users.findOneAndDelete(
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{ email: "alice@example.com" }
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)
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```
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## Bulk Operations
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```javascript
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// Ordered bulk write (stops on first error)
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db.users.bulkWrite([
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{ insertOne: { document: { name: "Alice" } } },
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{ updateOne: {
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filter: { name: "Bob" },
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update: { $set: { age: 25 } }
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}},
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{ deleteOne: { filter: { name: "Charlie" } } }
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])
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// Unordered (continues on errors)
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db.users.bulkWrite(operations, { ordered: false })
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```
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## Best Practices
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1. **Use projection** to return only needed fields
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2. **Create indexes** on frequently queried fields
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3. **Use updateMany** carefully (can affect many documents)
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4. **Use upsert** for "create or update" patterns
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5. **Use atomic operators** ($inc, $push) for concurrent updates
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6. **Avoid large arrays** in documents (embed vs reference)
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7. **Use findAndModify** for atomic read-modify-write
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8. **Batch operations** with insertMany/bulkWrite for efficiency
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