We added an additional check on the fields used as key, so that TTL is ignored for CompoundIndex with more than one field (which effectively renders it useless on @CompoundIndex at all).
Prior to this change potentially invalid index structures would have been created for e.g. @CompoundIndex(def = "{'foo': 1, 'bar': 1}", expireAfterSeconds=10) leading to MongoDB not being able to clean up the indexes (logs: "ERROR: key for ttl index can only have 1 field")
This fix is related to https://jira.mongodb.org/browse/SERVER-10075.
Original pull request: #196.
118f007ca6