# MongoDB Quick Reference

*CRUD, queries, aggregation, indexes, schema design*

> Source: MongoDB Documentation (mongodb.com/docs) · MIT

## Connecting

### Connection String

```
mongosh "mongodb://localhost:27017"
mongosh "mongodb://user:pass@host:27017/mydb"
mongosh "mongodb+srv://user:pass@cluster.mongodb.net/mydb"
```

### Driver Connection (Node.js)

```
const { MongoClient } = require('mongodb');
const client = new MongoClient(uri);
await client.connect();
const db = client.db('mydb');
```

## Databases & Collections

### Database Operations

```
show dbs
use mydb
db.dropDatabase()
```

### Collection Operations

```
db.createCollection("users")
show collections
db.users.drop()
```

### Capped Collection

```
db.createCollection("logs", {
  capped: true, size: 10485760, max: 5000
})
```

## CRUD Operations

### Insert

```
db.users.insertOne({ name: "Alice", age: 30 })
db.users.insertMany([
  { name: "Bob", age: 25 },
  { name: "Carol", age: 28 }
])
```

### Find

```
db.users.findOne({ name: "Alice" })
db.users.find({ age: { $gte: 25 } })
db.users.find({}, { name: 1, _id: 0 })
db.users.find().sort({ age: -1 }).limit(10)
```

### Update

```
db.users.updateOne(
  { name: "Alice" },
  { $set: { age: 31, city: "Boston" } }
)
db.users.updateMany(
  { active: false },
  { $set: { archived: true } }
)
```

### Delete

```
db.users.deleteOne({ name: "Alice" })
db.users.deleteMany({ active: false })
```

### Replace & Upsert

```
db.users.replaceOne(
  { name: "Alice" },
  { name: "Alice", age: 32, city: "NYC" }
)
db.users.updateOne(
  { email: "a@ex.com" },
  { $set: { name: "Alice" } },
  { upsert: true }
)
```

## Query Operators

### Comparison

| Command | Description |
|---------|-------------|
| `$eq / $ne` | Equal / not equal |
| `$gt / $gte` | Greater than / greater or equal |
| `$lt / $lte` | Less than / less or equal |
| `$in / $nin` | In array / not in array |

### Logical

| Command | Description |
|---------|-------------|
| `$and` | All conditions must match |
| `$or` | At least one condition matches |
| `$not` | Negates a condition |
| `$exists` | Field exists (true/false) |
| `$regex` | Regular expression match |

### Update Operators

| Command | Description |
|---------|-------------|
| `$set` | Set field value |
| `$unset` | Remove field |
| `$inc` | Increment numeric value |
| `$push / $pull` | Add / remove array element |
| `$addToSet` | Add to array if not present |
| `$rename` | Rename a field |

## Aggregation

### Pipeline Stages

| Command | Description |
|---------|-------------|
| `$match` | Filter documents (like WHERE) |
| `$group` | Group and aggregate |
| `$project` | Reshape documents (like SELECT) |
| `$sort` | Sort results |
| `$limit / $skip` | Pagination |
| `$lookup` | Left outer join with another collection |
| `$unwind` | Deconstruct array into documents |

### Aggregation Example

```
db.orders.aggregate([
  { $match: { status: "completed" } },
  { $group: {
      _id: "$customer_id",
      total: { $sum: "$amount" },
      count: { $sum: 1 }
  }},
  { $sort: { total: -1 } },
  { $limit: 10 }
])
```

## Indexes

### Create & Drop

```
db.users.createIndex({ email: 1 }, { unique: true })
db.users.createIndex({ name: 1, age: -1 })
db.users.createIndex({ location: "2dsphere" })
db.users.dropIndex("email_1")
```

### Index Types

| Command | Description |
|---------|-------------|
| `Single field` | Index on one field ({ name: 1 }) |
| `Compound` | Multiple fields ({ a: 1, b: -1 }) |
| `Text` | Full-text search ({ field: 'text' }) |
| `2dsphere` | Geospatial queries |
| `TTL` | Auto-expire documents after time |

### Index Info

```
db.users.getIndexes()
db.users.find({ name: "Alice" }).explain()
```

## Schema Design

### Embedding vs Referencing

| Command | Description |
|---------|-------------|
| `Embed` | 1:1 or 1:few, data read together |
| `Reference` | 1:many, data accessed independently |
| `Embed` | Sub-document rarely exceeds 16 MB |
| `Reference` | Many-to-many relationships |

### Schema Validation

```
db.createCollection("users", {
  validator: { $jsonSchema: {
    bsonType: "object",
    required: ["name", "email"],
    properties: {
      name: { bsonType: "string" },
      email: { bsonType: "string" }
}}}})
```

## Replication

### Replica Set Concepts

| Command | Description |
|---------|-------------|
| `Primary` | Receives all writes |
| `Secondary` | Replicates from primary, can serve reads |
| `Arbiter` | Votes in elections, holds no data |

### Replica Set Commands

```
rs.initiate()
rs.add("mongo2:27017")
rs.addArb("mongo3:27017")
rs.status()
rs.conf()
```

## Common Patterns

### Transactions

```
const session = client.startSession();
session.startTransaction();
await db.collection("accounts").updateOne(
  { _id: 1 }, { $inc: { bal: -100 } }, { session });
await db.collection("accounts").updateOne(
  { _id: 2 }, { $inc: { bal: 100 } }, { session });
await session.commitTransaction();
```

### Bulk Write

```
db.users.bulkWrite([
  { insertOne: { document: { name: "Dan" } } },
  { updateOne: {
      filter: { name: "Alice" },
      update: { $set: { age: 31 } }
  }},
  { deleteOne: { filter: { name: "old" } } }
])
```

### Change Streams

```
const stream = db.collection("orders")
  .watch([{ $match: { "fullDocument.status": "new" } }]);
stream.on("change", (change) => {
  console.log(change.fullDocument);
});
```

## mongosh Commands

### Shell Helpers

| Command | Description |
|---------|-------------|
| `show dbs` | List databases |
| `show collections` | List collections in current db |
| `db.stats()` | Database statistics |
| `db.collection.stats()` | Collection statistics |
| `db.collection.countDocuments({})` | Count documents |
| `db.collection.distinct('field')` | Distinct values for a field |

### Export & Import

```
mongoexport --db=mydb --collection=users \
  --out=users.json
mongoimport --db=mydb --collection=users \
  --file=users.json
mongodump --db=mydb --out=/backup/
mongorestore --db=mydb /backup/mydb/
```