Q1. How would you model an e-commerce cart in MongoDB?
Typically, you'd have a 'carts' collection where each cart belongs to a user. Cart document contains items array with productId, quantity, price at time of addition. You might also have a separate 'products' collection. This allows flexibility and easy updates.
Q2. What would a cart schema look like in Mongoose?
const cartSchema = new mongoose.Schema({
userId: { type: mongoose.Schema.Types.ObjectId, ref: 'User', required: true },
items: [{
productId: { type: mongoose.Schema.Types.ObjectId, ref: 'Product' },
name: String,
price: Number,
quantity: { type: Number, default: 1 }
}],
updatedAt: { type: Date, default: Date.now }
});
Q3. How do you add an item to the cart?
Find the cart by userId, then either push new item or update quantity:
const cart = await Cart.findOne({ userId });
const existing = cart.items.find(i => i.productId.equals(productId));
if (existing) {
existing.quantity += quantity;
} else {
cart.items.push({ productId, name, price, quantity });
}
await cart.save();
Q4. How do you calculate the total price?
Add a virtual or method to the schema:
cartSchema.virtual('total').get(function() {
return this.items.reduce((sum, item) => sum + (item.price * item.quantity), 0);
});
Q5. How do you handle stock validation during checkout?
During checkout, you need to check product quantities. This might involve transactions (if using replica sets) or two-phase commits. In a simple app, you can:
const session = await mongoose.startSession();
session.startTransaction();
try {
// check and update stock for each item
await session.commitTransaction();
} catch {
await session.abortTransaction();
} finally {
session.endSession();
}