Check out example codes for "how to add a new propety into all documents in mongodb". It will help you in understanding the concepts better.

Code Example 1

db.myCollection.update({}, {$set: {"isOpen": false}}, false, true)

Code Example 2

db.users.update({}, { "$set" : { "age": 30 }}, false,true)
// users: collection name, age: new property
//false it's upsert argument, it tells mongo to not insert a new document when no match is found
// true it's multi argument, it tells mongo to update multiple documents that meet the query criteria

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