Creating, Updating, and Deleting Documents(Chapter 3 of MongoDB The Definitive G
时间：2010-10-01 来源：Ray Z
"$set" sets the value of a key. If the key does not yet exist, it will be created. This can be handy for updating schema or adding user-defined keys.
The "$inc" modifier can be used to change the value for an existing key or to create a new key if it does not already exist.
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