MongoDB $or Operator

What Is $or?

$or means logical OR. A document matches when at least one condition in the array is true. In aggregation expressions it returns true / false the same way.

$or = any condition true → match / true

Query / $match:
  {
    $or: [
      { price: { $lt: 10 } },
      { inStock: false }
    ]
  }

Aggregation expression:
  {
    $or: [
      { $gt: [ "$price", 20 ] },
      { $lt: [ "$price", 10 ] }
    ]
  }

empty array → false (expression form)
prefer $in when OR-ing equalities on ONE field

How to Use $or in MongoDB

List conditions in an array. Keep the document (or return true) if any one passes.

1. For filtering: use find() or $match with { $or: [ {...}, {...} ] }.
2. For a boolean: use $addFields / $project with expression-style comparisons inside $or.
3. Put each clause in its own object (query form) or as a boolean expression (agg form).
4. If every clause is equality on the same field, switch to $in.

Pseudocode

// Filter mode
for each document:
    if any condition in $or array is true:
        keep document
    else:
        drop document

// Expression mode
for each document:
    result = (cond1 OR cond2 OR ...)
    emit { ..., field: result }

Cheat Sheet

Keep these facts handy while writing OR conditions.

Live Preview

Watch $or keep a product when price < 10 or inStock is false.

item = "Pen"
price < 10 → true
inStock false → false
$or → true (kept)

Worked Walkthrough — Cheap or Out of Stock

Keep products that are under $10 or out of stock.

Documents:
  { name: "Pen",  price: 5,  inStock: true }
  { name: "Book", price: 15, inStock: false }
  { name: "Lamp", price: 25, inStock: true }

$or: [ price < 10 , inStock == false ]

Pen:  5 < 10 ✓           → keep
Book: inStock false ✓    → keep
Lamp: neither            → drop

Result: Pen and Book

MongoDB Examples

We use three products. First filter with query $or, then build a boolean flag in aggregation. Run these in mongosh after inserting the sample data once.

db.products.insertMany([
  { _id: 1, name: "Pen",  price: 5,  inStock: true },
  { _id: 2, name: "Book", price: 15, inStock: false },
  { _id: 3, name: "Lamp", price: 25, inStock: true }
]);

db.products.find()

[
  { _id: 1, name: "Pen",  price: 5,  inStock: true },
  { _id: 2, name: "Book", price: 15, inStock: false },
  { _id: 3, name: "Lamp", price: 25, inStock: true }
]

$or: cheap (< 10)  OR  out of stock

  Pen  → cheap ✓
  Book → out of stock ✓
  Lamp → neither ✗

db.products.find({
  $or: [
    { price: { $lt: 10 } },
    { inStock: false }
  ]
});

[
  { _id: 1, name: "Pen",  price: 5,  inStock: true },
  { _id: 2, name: "Book", price: 15, inStock: false }
]

db.products.aggregate([
  {
    $addFields: {
      extremePrice: {
        $or: [
          { $gt: ["$price", 20] },
          { $lt: ["$price", 10] }
        ]
      }
    }
  }
]);

[
  { _id: 1, name: "Pen",  price: 5,  inStock: true,  extremePrice: true },
  { _id: 2, name: "Book", price: 15, inStock: false, extremePrice: false },
  { _id: 3, name: "Lamp", price: 25, inStock: true,  extremePrice: true }
]

Performance Notes

Put selective $or filters in early $match stages, and keep clauses index-friendly when the collection is large.

Key Takeaways