When we query mongos 3.2 or 3.4 with a read preference and read concern, the command should be assembled like:
{ "$query": { "find": "test", "filter": {}, "readConcern": {"level": "majority"} }, "$readPreference": { "mode": "secondary" } }
However, the driver sends:
{ "$query": { "find": "test", "filter": {}, "readConcern": {"level": "majority"} }, "$readPreference": { "mode": "secondary" }, "readConcern": {"level": "majority"} }
mongos apparently ignores the outer readConcern.