MongoDB $in Operator

Example 1 — Sample Data

Imagine three stores. Each has a location and an in_stock list of fruit. We save them in a collection named fruit.

db.fruit.insertMany([
  { _id: 1, location: "24th Street", in_stock: ["apples", "bananas", "oranges"] },
  { _id: 2, location: "36th Street", in_stock: ["bananas", "pears"] },
  { _id: 3, location: "82nd Street", in_stock: ["apples", "oranges"] }
]);

How the Data Is Stored in MongoDB

After the insert, MongoDB keeps three documents in fruit. Run find() to see them:

db.fruit.find()

[
  { _id: 1, location: "24th Street", in_stock: ["apples", "bananas", "oranges"] },
  { _id: 2, location: "36th Street", in_stock: ["bananas", "pears"] },
  { _id: 3, location: "82nd Street", in_stock: ["apples", "oranges"] }
]

Read those documents like this:

Look for "bananas" in each list:

  24th: apples, bananas, oranges  → yes
  36th: bananas, pears            → yes
  82nd: apples, oranges           → no

$in asks only "present?" — not "where?" or "how many?"

How It Works

1. Insert once. insertMany adds three store documents.
2. MongoDB stores arrays as written. Each in_stock value is a list of strings.
3. find() shows the same shape. What you inserted is what you get back—ready for $in next.

Example 2 — Boolean Flag with Aggregation $in

Goal: add hasBananas: true/false without removing stores. Remember: value first, array second.

db.fruit.aggregate([
  {
    $addFields: {
      hasBananas: {
        $in: ["bananas", "$in_stock"]
      }
    }
  }
]);

[
  {
    _id: 1,
    location: "24th Street",
    in_stock: ["apples", "bananas", "oranges"],
    hasBananas: true
  },
  {
    _id: 2,
    location: "36th Street",
    in_stock: ["bananas", "pears"],
    hasBananas: true
  },
  {
    _id: 3,
    location: "82nd Street",
    in_stock: ["apples", "oranges"],
    hasBananas: false
  }
]

How It Works

1. Read the array. "$in_stock" is the list for each store.
2. Search for bananas. $in: ["bananas", "$in_stock"] returns true when that string is present.
3. 82nd Street fails. Its list has apples and oranges only.
4. Keep all rows. $addFields only adds the boolean flag.

Example 3 — Filter with $match

Same idea, but keep only stores that stock bananas. Wrap the aggregation expression in $expr.

db.fruit.aggregate([
  {
    $match: {
      $expr: {
        $in: ["bananas", "$in_stock"]
      }
    }
  }
]);

[
  { _id: 1, location: "24th Street", in_stock: ["apples", "bananas", "oranges"] },
  { _id: 2, location: "36th Street", in_stock: ["bananas", "pears"] }
]

How It Works

1. Same stored documents. Still using the three stores from Example 1.
2. Same membership test. Only rows where bananas appear survive.
3. $expr unlocks the expression. Aggregation $in inside $match needs $expr.
4. Query-style tip. For a scalar field against a list of literals, prefer { status: { $in: ["A", "B"] } }—often index-friendly.