MongoDB $filter Operator

Example 1 — Sample Data

Imagine one cart named Office with four line items and different prices.

db.carts.insertOne({
  _id: 1,
  name: "Office",
  items: [
    { name: "Pen",  price: 80 },
    { name: "Book", price: 120 },
    { name: "Mug",  price: 45 },
    { name: "Lamp", price: 200 }
  ]
});

How the Data Is Stored in MongoDB

After the insert, MongoDB keeps one document in carts. Run find() to see it:

db.carts.find()

{
  _id: 1,
  name: "Office",
  items: [
    { name: "Pen",  price: 80 },
    { name: "Book", price: 120 },
    { name: "Mug",  price: 45 },
    { name: "Lamp", price: 200 }
  ]
}

Read that document like this:

items array:

  Pen 80 | Book 120 | Mug 45 | Lamp 200
              |
              v  $filter (price >= 100)
              |
         Book 120 | Lamp 200

How It Works

1. Insert once. insertOne adds a single cart document to carts.
2. Store an array. items holds embedded objects—exactly what $filter expects as input.
3. find() shows the same shape. Ready for the filter examples next.

Example 2 — Filter by Price

Goal: keep only items where price >= 100.

db.carts.aggregate([
  {
    $addFields: {
      expensive: {
        $filter: {
          input: "$items",
          as: "item",
          cond: { $gte: [ "$$item.price", 100 ] }
        }
      }
    }
  }
]);

{
  _id: 1,
  name: "Office",
  items: [
    { name: "Pen",  price: 80 },
    { name: "Book", price: 120 },
    { name: "Mug",  price: 45 },
    { name: "Lamp", price: 200 }
  ],
  expensive: [
    { name: "Book", price: 120 },
    { name: "Lamp", price: 200 }
  ]
}

How It Works

1. Read the array. input: "$items" points at the stored cart lines.
2. Name each element. as: "item" lets you use "$$item" in cond.
3. Test the price. $gte keeps Book and Lamp; Pen and Mug are dropped.
4. Add a new field. $addFields stores the result in expensive without deleting items.

Example 3 — Filter with Limit

Same condition, but keep only the first matching item (limit: 1).

db.carts.aggregate([
  {
    $project: {
      name: 1,
      topExpensive: {
        $filter: {
          input: "$items",
          as: "item",
          cond: { $gte: [ "$$item.price", 100 ] },
          limit: 1
        }
      }
    }
  }
]);

{
  _id: 1,
  name: "Office",
  topExpensive: [
    { name: "Book", price: 120 }
  ]
}

How It Works

1. Same condition. Still keep items with price >= 100.
2. Stop early. limit: 1 returns only the first match in original order (Book before Lamp).
3. Project a small shape. Output shows cart name and the limited array.
4. Not a sort. Limit uses input order—it does not pick the highest price unless you sort first.