MongoDB $trunc Operator

Example 1 — Sample Data

Imagine three temperature sensors. Values include decimals and one negative reading so we can see toward-zero behavior.

db.readings.insertMany([
  { _id: 1, sensor: "Temp-A", value: 21.578 },
  { _id: 2, sensor: "Temp-B", value: 18.942 },
  { _id: 3, sensor: "Temp-C", value: -3.789 }
]);

How the Data Is Stored in MongoDB

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

db.readings.find()

[
  { _id: 1, sensor: "Temp-A", value: 21.578 },
  { _id: 2, sensor: "Temp-B", value: 18.942 },
  { _id: 3, sensor: "Temp-C", value: -3.789 }
]

Read each document like this:

How It Works

1. Insert once. insertMany adds three reading documents to readings.
2. Values stay precise. MongoDB stores the full decimals until you truncate them.
3. find() shows the same shape. What you inserted is what you get back—ready for $trunc next.

Example 2 — Truncate Decimals

Goal: keep two digits after the decimal without rounding up. 21.578 should become 21.57, not 21.58.

db.readings.aggregate([
  {
    $addFields: {
      truncated: { $trunc: ["$value", 2] }
    }
  }
]);

{
  _id: 1,
  sensor: "Temp-A",
  value: 21.578,
  truncated: 21.57
}
{
  _id: 2,
  sensor: "Temp-B",
  value: 18.942,
  truncated: 18.94
}
{
  _id: 3,
  sensor: "Temp-C",
  value: -3.789,
  truncated: -3.78
}

How It Works

1. Read the stored value. Temp-A has value: 21.578 from Example 1.
2. Keep two places. $trunc: ["$value", 2] drops digits after the second decimal.
3. No round-up. The next digit is 8, but $trunc still returns 21.57 (unlike $round).

Example 3 — Truncate to Integer

Same readings, but use place: 0 to drop the fractional part toward zero—especially clear on the negative value.

db.readings.aggregate([
  {
    $addFields: {
      asInt: { $trunc: ["$value", 0] },
      floored: { $floor: "$value" }
    }
  }
]);

{
  _id: 1,
  sensor: "Temp-A",
  value: 21.578,
  asInt: 21,
  floored: 21
}
{
  _id: 2,
  sensor: "Temp-B",
  value: 18.942,
  asInt: 18,
  floored: 18
}
{
  _id: 3,
  sensor: "Temp-C",
  value: -3.789,
  asInt: -3,
  floored: -4
}

How It Works

1. Same stored documents. Still using the three readings from Example 1.
2. Toward zero. $trunc turns -3.789 into -3.
3. Compare with floor. $floor turns the same value into -4 (toward −∞).
4. Easy check. Positive values match floor here; the negative row shows the difference.