How to merge multiple databases in mongodb

Question

Now there are DBA, DBB, DBC, and NewDB in an instance. How to import ABC into NewDB?
The structure is the same. I tried For Insert and InsertMany, but the data is close to 500W, and it won’t work after inserting a few items. And the efficiency is too low.
Is there any efficient way?

Answer 1

If it is a copy set, there is a way. It is not very intuitive, but we have renameCollection Command:
Note: Be sure to read the above link before doing it.

{ renameCollection: "<source_namespace>", to: "<target_namespace>", dropTarget: <true|false> }

Field	Type	Description
renameCollection	string	The namespace of the collection to rename. The namespace is a combination of the database name and the name of the collection.
to	string	The new namespace of the collection. If the new namespace specifies a different database, the renameCollection command copies the collection to the new database and drops the source collection.
dropTarget	boolean	Optional. If true, mongod will drop the target of renameCollection prior to renaming the collection. The default value is false.

Example:

use admin
db.runCommand( { renameCollection: "DBA.xxx", to: "NewDB.xxx" } )
db.runCommand( { renameCollection: "DBA.yyy", to: "NewDB.yyy" } )
...