MongoDB: How to merge all documents into a single document in an aggregation pipeline

Question

I have the current aggregation output as follows:

[
    {
        "courseCount": 14
    },
    {
        "registeredStudentsCount": 1
    }
]

The array has two documents. I would like to combine all the documents into a single document having all the fields in mongoDB

Answer 1

 db.collection.aggregate([
 {
   $group: {
    _id: 0,
     merged: {
    $push: "$$ROOT"
   }
   }
  },
 {
  $replaceRoot: {
  newRoot: {
    "$mergeObjects": "$merged"
   }
  }
 }
])

Explained:

1. Group the output documents in one field with push
2. Replace the document root with the merged objects

Plyaground