How to duplicate a document during aggregation in mongo?

user_9812143 0
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Is there any way to duplicate a document during aggregation in mongo? For example, I've got a "Person" schema that looks like that:

Person:

{
  _id: ObjectId,
  fullName: string,
  matches: number
}

I want to make an aggregation that returns all the persons and duplicates each person's document by the number of his matches.

for example for this collection:

[{
  _id: 1,
  fullName: "John Doe",
  matches: 1
},
{
  _id: 2,
  fullName: "Bla Bla",
  matches: 2
}]

The result will be:

[{
  _id: 1,
  fullName: "John Doe",
  matches: 1
},
{
  _id: 2,
  fullName: "Bla Bla",
  matches: 2
},
{
  _id: 2,
  fullName: "Bla Bla",
  matches: 2
}]

I know that I can add fields to the documents during the aggregation but didn't found anything about adding documents.

aggregation-framework mongodb
1 Answers
  • $map to iterate loop of range, $range start from 0 and end at matches number, it will return array of current root document
  • $unwind deconstruct matches array
  • $replaceRoot replace matches object to root
db.collection.aggregate([
  {
    $project: {
      matches: {
        $map: {
          input: { $range: [0, "$matches"] },
          in: "$$ROOT"
        }
      }
    }
  },
  { $unwind: "$matches" },
  { $replaceRoot: { newRoot: "$matches" } }
])

Playground

user_89871280
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