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Steps To Reproduce:

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First, shard the collection that you're map/reducing over.
Then, run a command like:

mongos> db.runCommand( { "mapreduce" : "test", map : "function() {}", reduce : "function() {}", out :
{inline:1}
, "$readPreference" :
{ mode : "primary"}
} )
{ "ok" : 0, "errmsg" : "unknown m/r field for sharding: $readPreference" }

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First, shard the collection that you're map/reducing over. Then, run a command like: mongos> db.runCommand( { "mapreduce" : "test", map : "function() {}", reduce : "function() {}", out : {inline:1} , "$readPreference" : { mode : "primary"} } ) { "ok" : 0, "errmsg" : "unknown m/r field for sharding: $readPreference" }

Running a mapReduce against mongos on a sharded collection with a driver that supports $readPreference produces a server error:

{ "serverUsed" : "localhost/127.0.0.1:30999" , "ok" : 0.0 , "errmsg" : "unknown m/r field for sharding: $readPreference"}

is related to

SERVER-7791 Commands wrapped in $query reports wrong error message for unrecognized commands