Why should you care?

Business rules like “discount must be less than price” or “stock % 2 == 0” need evaluation, not only equality.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.products.insertMany([
  { name: "Cable", price: 200, discount: 50 },
  { name: "Dock", price: 2000, discount: 2500 },
  { name: "Tape", price: 100, discount: 10 }
])
db.products.find({
  $expr: { $lt: ["$discount", "$price"] }
})
db.products.find({ price: { $mod: [100, 0] } })

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• $expr compares discount and price fields — Dock is excluded because discount > price.
• $mod: [100, 0] keeps prices divisible by 100.
• Field paths inside $expr use $fieldName.

Practice next

1. Insert the three products.
2. Run the $expr query and confirm Dock is out.
3. Run the $mod query.
4. Find where $subtract price-discount > 1000 using $expr.
5. Combine $expr with category equality.

Remember

$expr compares fields inside a document. $mod filters by remainder. Prefer simple operators when possible for indexes.

Promo sanity check

Before publishing deals, a job finds products where discount >= price using $expr.

Outcome: Broken promos never hit the storefront.