mongodb

MongoDB

Overview

MongoDB is a document database that stores data in flexible JSON-like documents. This skill covers schema design, queries, indexing, and aggregation pipelines.

Schema Design

Embedding vs Referencing

// EMBED when data is read together and has a 1:few relationship
{
  _id: ObjectId("..."),
  name: "John",
  addresses: [
    { street: "123 Main", city: "Springfield", type: "home" },
    { street: "456 Work", city: "Springfield", type: "work" }
  ]
}

// REFERENCE when data is many:many, frequently updated independently, or large
{
  _id: ObjectId("..."),
  name: "John",
  organization_id: ObjectId("...")  // Reference to organizations collection
}

Schema Validation

db.createCollection("users", {
  validator: {
    $jsonSchema: {
      bsonType: "object",
      required: ["email", "name", "createdAt"],
      properties: {
        email: { bsonType: "string", pattern: "^.+@.+$" },
        name: { bsonType: "string", maxLength: 100 },
        role: { enum: ["admin", "member", "viewer"] },
        createdAt: { bsonType: "date" }
      }
    }
  }
});

Query Patterns

CRUD

// Find with projection
const user = await db.collection('users').findOne(
  { email: "[email protected]" },
  { projection: { password_hash: 0 } }  // Exclude sensitive fields
);

// Update with operators
await db.collection('users').updateOne(
  { _id: userId },
  { $set: { name: "New Name" }, $currentDate: { updatedAt: true } }
);

// Upsert
await db.collection('metrics').updateOne(
  { date: today, type: "pageview" },
  { $inc: { count: 1 } },
  { upsert: true }
);

Aggregation Pipeline

const result = await db.collection('orders').aggregate([
  { $match: { status: "completed", createdAt: { $gte: thirtyDaysAgo } } },
  { $group: { _id: "$customerId", totalSpent: { $sum: "$total" }, orderCount: { $sum: 1 } } },
  { $sort: { totalSpent: -1 } },
  { $limit: 10 },
  { $lookup: { from: "customers", localField: "_id", foreignField: "_id", as: "customer" } },
  { $unwind: "$customer" },
  { $project: { name: "$customer.name", totalSpent: 1, orderCount: 1 } }
]).toArray();