ByDefKey returns a filter by a def key. It panics if the def path is not set. If you pass a ByDefKey filter to a store that's scoped to a specific repo/version/unit, then it will match all items in that repo/version/unit even if the key.Repo/key.CommitID/key.UnitType/key.Unit fields do not match (because stores do not "know" the repo/version/unit they store data for, and therefore they can't apply filter criteria for the level above them).