$replaceRoot in mongodb aggregation

Question

I have a collection like this:

{
    \"_id\" : ObjectId(\"5bd1686ba64b9206349284db\"),
    \"type\" : \"Package\",
    \"codeInstances\" : [ 
        {
                  


        

Answer 1

You just need to project for name and description instead of whole codeInstances. Check below

db.collection.aggregate([
  { $unwind: "$codeInstances" },
  { $match: { "codeInstances.name": "b", "type": "Package" }},
  { $project: {
      "name": "$codeInstances.name",
      "description": "$codeInstances.description",
      "_id": 0
  }}
])

Output:

[
  { "description": "b1", "name": "b" },
  { "description": "b2", "name": "b" },
  { "description": "b3", "name": "b" }
]

Answer 2

You can try below aggregation using $replaceRoot

db.collection.aggregate([
  { "$match": { "codeInstances.name": "b", "type": "Package" }},
  { "$unwind": "$codeInstances" },
  { "$match": { "codeInstances.name": "b", "type": "Package" }},
  { "$replaceRoot": { "newRoot": "$codeInstances" }}
])