How do I append Mongo DB aggregation results to an existing collection?

Question

I am trying to perform several insertions on an existent Mongo DB collection using the following code

db.dados_meteo.aggregate( [
                  { $match          


        

Answer 1

Starting Mongo 4.2, the new $merge aggregation operator (similar to $out) allows merging the result of an aggregation pipeline into the specified collection:

Given this input:

db.source.insert([
  { "_id": "id_1", "a": 34 },
  { "_id": "id_3", "a": 38 },
  { "_id": "id_4", "a": 54 }
])
db.target.insert([
  { "_id": "id_1", "a": 12 },
  { "_id": "id_2", "a": 54 }
])

the $merge aggregation stage can be used as such:

db.source.aggregate([
  // { $whatever aggregation stage, for this example, we just keep records as is }
  { $merge: { into: "target" } }
])

to produce:

// > db.target.find()
{ "_id" : "id_1", "a" : 34 }
{ "_id" : "id_2", "a" : 54 }
{ "_id" : "id_3", "a" : 38 }
{ "_id" : "id_4", "a" : 54 }

Note that the $merge operator comes with many options to specify how to merge inserted records conflicting with existing records.

In this case (with the default options), this:

• keeps the target collection's existing documents (this is the case of { "_id": "id_2", "a": 54 })

• inserts documents from the output of the aggregation pipeline into the target collection when they are not already present (based on the _id - this is the case of { "_id" : "id_3", "a" : 38 })

• replaces the target collection's records when the aggregation pipeline produces documents existing in the target collection (based on the _id - this is the case of { "_id": "id_1", "a": 12 } replaced by { "_id" : "id_1", "a" : 34 })