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https://docs.mongodb.org/manual/reference/method/db.collection.mapReduce/#mapreduce-out-mtd

下面是对mapreduce  output的说明

Output to a Collection with an Action

This option is only available when passing a collection that already exists to out. It is not available on secondary members of replica sets.

out: {
     
     
      : 
      
               [, db: 
       
        ]        [, sharded: 
        
          ]        [, nonAtomic: 
         
           ] }
         
        
       
      
     

When you output to a collection with an action, the out has the following parameters:

• <action>: Specify one of the following actions:

  • replace

    Replace the contents of the <collectionName> if the collection with the<collectionName> exists.

  • merge

    Merge the new result with the existing result if the output collection already exists. If an existing document has the same key as the new result, overwrite that existing document.

  • reduce

    Merge the new result with the existing result if the output collection already exists. If an existing document has the same key as the new result, apply the reduce function to both the new and the existing documents and overwrite the existing document with the result.

• db:

  Optional. The name of the database that you want the map-reduce operation to write its output. By default this will be the same database as the input collection.

• sharded:

  Optional. If true and you have enabled sharding on output database, the map-reduce operation will shard the output collection using the _id field as the shard key.

• nonAtomic:

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