How do I get all fields from multiple collections without any conditions in mongo DB

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aggregation

MongoDB

I have make a union of two collections without any conditions, and need to project all fields in both tables in one query. But with my query I am able to see the output of customer_id, name and concatination. The collection data which I am trying to project inside the union query is not showing.

I am not able to figure out the problem. Can anyone please help me on this?

What I have tried:

db.customer.aggregate(
        {
          "$unionWith": {
            "coll": "parent_child", "pipeline": [
              {
                "$project": {
                  "child_id":"$CUSTOMER_CHILD_ID",
                  "parent_id":"$CUSTOMER_PARENT_ID"
              }
            }
            ]
          }
        },
        {
          "$project":{
            "customer_id":"$CUSTOMER_ID",
            "name":"$NAME",
            "concatination":{"$concat":["$CLASS","$COMPANY_CODE"]}
          }
        }
      )