Running collmod on a sharded collection returns the following fields:

• raw
• ok
• $clusterTime
• operationTime

However, In order to guarantee output parity between replicaset and sharded cluster, the collmod coordinator enforces raw + single field output when working on an unsharded collection 

Example:

`
{collMod: "x", index: {keyPattern:

, expireAfterSeconds: 3600}
`
will return

• { raw,ok,$clusterTime,operationTime }

  on a sharded cluster for a sharded collection

• expireAfterSeconds_new,ok,$clusterTime,operationTime in a replica-set 
• raw, expireAfterSeconds_new,ok,$clusterTime,operationTime  on a unsharded collection in a sharded cluster

Unsplittable collection are now treated as sharded collection, and they are currently missing the *_new *_old fields that guaranteed the parity. 

The goal for the ticket is to reintroduced those fields in case of unsplittable collection. 

Note: You can easily replicate the issue by adding a {createUnsplittableCollection : x } in the setup phase of mongos_unsharded_commands_interface_parity_with_replica_set.js