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I have a program where tasks get done in "batches". The top-level view of my documents look like this:

{
  _id: "xxx",
  status: "done",
  startedAt: 123456,
  finishedAt: 789101112,
  tasks: [...] // Huge array of deeply nested objects
}

Is it possible to construct a query for mongodb that returns only the length of the tasks array in this example?

Desired result:

{
  _id: "xxx",
  status: "done",
  startedAt: 123456,
  finishedAt: 789101112,
  numTasks: 9999
}

Thank you in advance for any advice you can give.

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1 Answer 1

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You can use $size

db.collection.aggregate([
  {
    $addFields: {
      numTasks: {
        "$size": "$tasks"
      }
    }
  }
])

Working Mongo playground

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1 Comment

Spot on! Thank you for your advice. And thanks for the mongo playground link, this is gonna be a huge help in the future!

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