MongoDB 相当于 Python 的 enumerate()

Question

I'm searching for a MongoDB aggregation to replace an array

['a', 'b', 'c']

by something like

[ {"i":1,"element":"a"}, {"i":2,"element":"b"}, {"i":3,"element":"c"} ]

Thanks

Answer 1

• $map to iterate loop of range from 0 to length of lyrics array using $range
• $add to add 1 in current index because range is started from 0
• $arrayElemAt to get specific element from lyrics array

db.collection.aggregate([
  {
    $set: {
      lyrics: {
        $map: {
          input: { $range: [0, { $size: "$lyrics" }] },
          in: {
            i: { $add: ["$$this", 1] },
            element: { $arrayElemAt: ["$lyrics", "$$this"] }
          }
        }
      }
    }
  }
])

Playground