In earlier versions of MongoDB, you could use the copyDB command or its helper method, db.copyDatabase() to copy a database.
MongoDB has since deprecated these. Also, starting in version 4.2, MongoDB has removed the copydb command, and therefore also the db.copyDatabase() method, which means that you can’t use them even if you wanted to if you’re using MongoDB 4.2 or later.
Fortunately, there’s another way to copy a MongoDB database.
In earlier versions of MongoDB, you could use the db.collection.copyTo() method to copy a collection.
MongoDB has since deprecated that method. Also, starting in version 4.2, MongoDB has removed the eval command. Given db.collection.copyTo() wraps eval, this means that you can’t use either of them if you’re using MongoDB 4.2 or later.
Fortunately, there’s another way to copy a MongoDB collection.
If you’ve backed up a database using the mongodump utility, you can restore it using the mongorestore utility.
The mongorestore utility loads data from either a binary database dump created by mongodump or the standard input into a mongod or mongos instance.
There are several ways to back up a database in MongoDB. One quick way to back up a database is to use the mongodump tool.
mongodump reads data from a MongoDB database and creates high fidelity BSON files which the mongorestore utitlity can use to restore a MongoDB database.
With mongodump, you can back up a collection, a database, or all databases.
In MongoDB, the distinct aggregation command finds the distinct values for a specified field across a single collection.
It returns a document that contains an array of the distinct values, as well as an embedded document with query statistics and the query plan.
In MongoDB, the db.collection.distinct() method finds the distinct values for a specified field across a single collection or view and returns the results in an array.
Distinct values are those with redundant duplicates removed. Distinct values are unique values. For example, if you have 2 or 3 documents with the same value, the distinct command will return just one value.
db.collection.distinct() is a shell wrapper method for the distinct command.
In MongoDB the $sortByCount aggregation pipeline stage groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.
Each group is output in its own document, which consists of two fields:
_id field containing the distinct grouping value, andcount field containing the number of documents belonging to that grouping.The documents are sorted by count in descending order.
In MongoDB the $count aggregation operator passes a document to the next stage in the aggregation pipeline that contains a count of the number of documents input to the current stage.
In MongoDB the cursor.count() method counts the number of documents referenced by a cursor..
Note that it doesn’t actually perform the query. It simply counts and returns the number of results that would be returned by the query.
The cursor.count() method is a wrapper for the count command.