When using shardcollection with a key that is a prefix of an existing compound index, one gets a message 'please create an index over the sharding key before sharding.', even though the compound index could have been used for sharding.

For instance:

db.Test.save(

)
db.Test.ensureIndex (

)

db.runCommand({shardcollection: "test.Test", key: {a:1}}) << use compound index on a,b here instead of giving error message