Mongoose

MongoDB reads every document in a collection if there is no index — a collection scan. At 10 million documents, that is slow. Indexes fix this. Single-Field Index // Create an index on the email field db.users.createIndex({ email: 1 }) // 1 = ascending, -1 = descending // The query now uses the index db.users.find({ email: "alex@example.com" }) // Unique index — enforces uniqueness db.users.createIndex({ email: 1 }, { unique: true }) Compound Index Index multiple fields together when you filter or sort on more than one: ...

find() gets documents. The aggregation pipeline transforms them. It is MongoDB’s answer to SQL GROUP BY, JOIN, and window functions. How the Pipeline Works Documents flow through a sequence of stages. Each stage takes the output of the previous one as input. db.orders.aggregate([ { $match: { status: "delivered" } }, // Stage 1: filter { $group: { _id: "$user_id", total: { $sum: "$amount" } } }, // Stage 2: group { $sort: { total: -1 } }, // Stage 3: sort { $limit: 10 } // Stage 4: limit ]) $match — Filter Documents // Equivalent to WHERE in SQL db.orders.aggregate([ { $match: { status: "delivered", createdAt: { $gte: ISODate("2026-01-01"), $lt: ISODate("2027-01-01") } } } ]) Put $match as early as possible. It reduces the number of documents processed by later stages. ...

MongoDB’s flexible schema is a strength and a trap. You can store anything, but bad data modeling causes slow queries and bloated documents. Good modeling matches your access patterns. The Key Question: Embed or Reference? In SQL, you normalize data into separate tables and join them. In MongoDB, you have a choice: embed related data in the same document, or store it separately and reference it. Embed when: Data is always accessed together Child data belongs to only one parent The embedded data is bounded in size Reference when: ...

MongoDB stores data as documents — JSON-like objects with flexible structure. No fixed schema. No joins required for embedded data. This tutorial gets you up and running with MongoDB 8.0. Setup with Docker docker run -d \ --name mongodb \ -p 27017:27017 \ -e MONGO_INITDB_ROOT_USERNAME=admin \ -e MONGO_INITDB_ROOT_PASSWORD=password \ mongodb/mongodb-community-server:8.0-ubi8 Connect with mongosh: mongosh "mongodb://admin:password@localhost:27017" mongosh Basics // Show databases show dbs // Switch to (or create) a database use myapp // Show collections show collections // Create a collection explicitly (optional — auto-created on first insert) db.createCollection("users") Documents and Collections In MongoDB: ...