Query Operators, Projections and Sorting

What you'll build

Catalogue queries a shop needs: products in a price range, products with a tag, in-stock items from several categories, a case-insensitive name search, and a sorted, limited result with only the fields the page shows.

The idea

• Comparison: $gt, $gte, $lt, $lte, $ne, $in, $nin. Logical: $and (implicit with several fields), $or, $not.
• Arrays: matching a value matches any element; $all requires several; $elemMatch applies several conditions to the same element.
• Projection ({ name: 1, price: 1, _id: 0 }) returns only listed fields.
• sort, limit, skip shape results; sort on indexed fields for large collections.

The code

db.products.insertMany([
  { _id: 1, name: "Mug", category: "kitchen", price: 8.5, stock: 40, tags: ["ceramic", "gift"] },
  { _id: 2, name: "Teapot", category: "kitchen", price: 24, stock: 5, tags: ["cast-iron"] },
  { _id: 3, name: "Kettle", category: "appliances", price: 31.9, stock: 0, tags: ["electric"] },
  { _id: 4, name: "Tea sampler", category: "food", price: 12, stock: 120, tags: ["gift", "organic"] },
]);

const show = (label, cursor) => print(label + ":", JSON.stringify(cursor.toArray()));

show("10 to 30", db.products.find({ price: { $gte: 10, $lte: 30 } }, { _id: 0, name: 1, price: 1 }));
show("gift tag", db.products.find({ tags: "gift" }, { _id: 0, name: 1 }));
show("in stock, kitchen or food", db.products.find(
  { stock: { $gt: 0 }, category: { $in: ["kitchen", "food"] } }, { _id: 0, name: 1 }));
show("name contains 'tea'", db.products.find({ name: { $regex: "tea", $options: "i" } }, { _id: 0, name: 1 }));
show("cheap OR out of stock", db.products.find({ $or: [{ price: { $lt: 10 } }, { stock: 0 }] }, { _id: 0, name: 1 }));
show("top 2 by price", db.products.find({}, { _id: 0, name: 1, price: 1 }).sort({ price: -1 }).limit(2));

10 to 30: [{"name":"Teapot","price":24},{"name":"Tea sampler","price":12}]
gift tag: [{"name":"Mug"},{"name":"Tea sampler"}]
in stock, kitchen or food: [{"name":"Mug"},{"name":"Teapot"},{"name":"Tea sampler"}]
name contains 'tea': [{"name":"Teapot"},{"name":"Tea sampler"}]
cheap OR out of stock: [{"name":"Mug"},{"name":"Kettle"}]
top 2 by price: [{"name":"Kettle","price":31.9},{"name":"Teapot","price":24}]

Common mistakes

• Unanchored, case-insensitive regex searches on large collections: they can't use indexes efficiently. Use an anchored prefix or Atlas Search / text indexes.
• Fetching whole documents for list pages; project only the fields you render.
• Deep pagination with skip, which still walks over skipped documents (see the pagination lesson).

Key points

• Combine comparison, logical and array operators in one filter document.
• Project only the fields you need.
• Sort and limit on the server, ideally on indexed fields.

Further reading: Query and projection operators · Query arrays