Restoring single collection in an existing mongodb

If you want to restore a single collection then you have to specifiy the dump file of the collection. The dump file of the collection is found in the 'dump/dbname/' folder. So assuming your dump folder is in your current working directory, the command would go something like -

mongorestore --db mydbname --collection mycollection dump/mydbname/mycollection.bson

I think this is now done with the --nsInclude option:

mongorestore --nsInclude test.purchaseorders dump/

dump/ is the folder with your mongodump data, test is the db, and purchaseorders is the collection.

https://docs.mongodb.com/manual/reference/program/mongorestore/