create a new collection from existing collection's object array type field in mongodb?

Question

I would like create a new collection from an existing collection. The structure of document is like this.

{"_id" : "ObjectId('1234')",
  "field2" : "value2",
  "field3" : [
        {"field3-1": "value3", 
         "field3-2":"value4", 
         "field3-3":"value5", 
         "field3-4":"value6"},
         {"field3-1": "value7", 
         "field3-2":"value8", 
         "field3-3":"value9", 
         "field3-4":"value10"}
    ]
}

I would like a new collection that has field3-1, field3-2, field3-3 and field3-4 like the following.

{
    "source_id": "1234",
    "field3-1": "value3",
    "field3-2":"value4", 
    "field3-3":"value5", 
    "field3-4":"value6"
},
{
    "source_id": "1234",
    "field3-1": "value7", 
    "field3-2":"value8", 
    "field3-3":"value9", 
    "field3-4":"value10"
}

I tried using export by choosing only field3, it created the file like the following when I selected the type as csv, which I can't export into any collection.

"[
"{
    ""field3-1"": ""value3"",
    ""field3-2"":""value4"", 
    ""field3-3"":""value5"", 
    ""field3-4"":""value6""
}",
"{
    ""field3-1"": ""value7"",
    ""field3-2"":""value8"", 
    ""field3-3"":""value9"", 
    ""field3-4"":""value10""
}"
]"

I have almost 20 million documents in the existing collection, what could be the efficient way to achieve it? If anyone has an experience, could you give some advices?

Answer 1

Simply do an $unwind. $replaceRoot to move the sub-documents to root level. $out to a new collection.

db.collection.aggregate([
  {
    "$unwind": "$field3"
  },
  {
    "$replaceRoot": {
      "newRoot": {
        "$mergeObjects": [
          "$field3",
          {
            // fields you want to keep
            "source_id": "$_id"
          }
        ]
      }
    }
  },
  {
    "$out": {
      "db": "output-db",
      "coll": "output-collection"
    }
  }
])

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