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Find a Shard Key

Every shared collection has a shard key. To display the shard key, connect to a mongos instance and run the db.printShardingStatus() method:

db.printShardingStatus()

The output resembles:

<dbname>.<collection>
   shard key: { <shard key> : <1 or hashed> }
   unique: <boolean>
   balancing: <boolean>
   chunks:
      <shard name1> <number of chunks>
      <shard name2> <number of chunks>
      ...
   { <shard key>: <min range1> } -->> { <shard key> : <max range1> } on : <shard name> <last modified timestamp>
   { <shard key>: <min range2> } -->> { <shard key> : <max range2> } on : <shard name> <last modified timestamp>
   ...
   tag: <tag1>  { <shard key> : <min range1> } -->> { <shard key> : <max range1> }
   ...

For more details on the db.printShardingStatus() output, see the sharded collection section on the sh.status() page.