Documents and Collections

MongoDB organizes data in a simple hierarchy: Database → Collection → Document.

The Hierarchy

flowchart TB
    subgraph Server[MongoDB Server]
        subgraph DB1[Database: ecommerce]
            C1[Collection: users]
            C2[Collection: products]
            C3[Collection: orders]
        end
        subgraph DB2[Database: blog]
            C4[Collection: posts]
            C5[Collection: comments]
        end
    end

    C1 --> D1[Document: {name: \"Alice\"}]
    C1 --> D2[Document: {name: \"Bob\"}]
    C2 --> D3[Document: {name: \"Laptop\", price: 75000}]
    C2 --> D4[Document: {name: \"Mouse\", price: 1200}]

    style Server fill:#1e293b,color:#fff
    style DB1 fill:#7c3aed,color:#fff
    style DB2 fill:#7c3aed,color:#fff
    style C1 fill:#3b82f6,color:#fff
    style C2 fill:#3b82f6,color:#fff
    style C3 fill:#3b82f6,color:#fff
    style C4 fill:#3b82f6,color:#fff
    style C5 fill:#3b82f6,color:#fff

Database

A MongoDB server can hold multiple databases. Each database is isolated (separate files on disk).

use myapp        // switch to (or create) database called "myapp"
db               // shows current database
show dbs         // list all databases

Collection

A collection is like a table — a group of related documents. But unlike SQL tables, collections don’t enforce a schema.

show collections                  // list collections in current db
db.createCollection("users")      // explicitly create a collection
db.users.drop()                   // delete a collection

💡 Collections are created automatically when you first insert data. You rarely need createCollection().

Document

A document is a single record, stored as BSON (Binary JSON). Think of it like a JavaScript object.

{
  "_id": ObjectId("64f1b2c3d4e5f6a7b8c9d0e1"),
  "name": "Alice Johnson",
  "email": "alice@example.com",
  "age": 28,
  "address": {
    "city": "Mumbai",
    "state": "Maharashtra"
  },
  "tags": ["admin", "verified"],
  "createdAt": ISODate("2024-01-15T10:30:00Z")
}

The `_id` Field

Every document must have a unique _id field. If you don’t provide one, MongoDB auto-generates an ObjectId.

flowchart LR
    subgraph ObjectId[ObjectId — 12 bytes]
        T[4 bytes<br/>Timestamp]
        R[5 bytes<br/>Random Value]
        C[3 bytes<br/>Counter]
    end

    ObjectId --> Example[Example: 64f1b2c3 d4e5f6a7b8 c9d0e1]

    style ObjectId fill:#7c3aed,color:#fff
    style T fill:#3b82f6,color:#fff
    style R fill:#059669,color:#fff
    style C fill:#f59e0b,color:#fff

// Auto-generated ObjectId
{ _id: ObjectId("64f1b2c3d4e5f6a7b8c9d0e1") }

// You can also use a custom _id
{ _id: "user_alice_001", name: "Alice" }
{ _id: 42, name: "Bob" }

// Extract timestamp from ObjectId
const id = new ObjectId("64f1b2c3d4e5f6a7b8c9d0e1");
id.getTimestamp(); // 2023-09-01T... (when it was created)

// Search by ObjectId (must use ObjectId(), not plain string)
db.users.findOne({ _id: ObjectId("64f1b2c3d4e5f6a7b8c9d0e1") })

BSON vs JSON

MongoDB stores data as BSON (Binary JSON) internally. You write queries in JSON-like syntax, and MongoDB converts automatically.

Feature	JSON	BSON
Format	Text (human-readable)	Binary (machine-readable)
Speed	Slower to parse	Faster to parse
Data types	Limited (string, number, bool, array, object, null)	Extended (Date, ObjectId, Binary, Decimal128, etc.)
Size	Smaller text	Slightly larger binary
Used for	API responses, config files	MongoDB internal storage

MongoDB Data Types

{
  _id:         ObjectId("64f1b2c3..."),     // Unique identifier
  name:        "Alice",                     // String
  age:         28,                          // Number (double by default)
  isActive:    true,                        // Boolean
  salary:      NumberDecimal("75000.50"),   // Decimal (precise)
  count:       NumberInt(42),               // 32-bit integer
  bigNum:      NumberLong(9007199254740993),// 64-bit integer
  createdAt:   ISODate("2024-01-15"),       // Date
  tags:        ["admin", "user"],           // Array
  address:     { city: "Mumbai" },          // Embedded object
  metadata:    null,                        // Null
  binaryData:  BinData(...),                // Binary data
}

In Simple Words

Database holds collections (like a folder)
Collection holds documents (like a table, but schema-free)
Document is a single record (like a JSON object)
Every document has a unique _id — usually an auto-generated ObjectId
MongoDB stores data as BSON (binary JSON) with support for more data types

Next: CRUD Operations →

Documents and Collections