PersistEntityGraph specifies type of single graph selector

src/Database/Persist/Local/Class/PersistEntityGraph.hs

@@ -23,6 +23,7 @@ import Prelude
 import Database.Persist
 
 class (PersistEntity n, PersistEntity e) => PersistEntityGraph n e where
+    type PersistEntityGraphSelector n e
     sourceParam :: e -> Key n
     sourceField :: EntityField e (Key n)
     destParam   :: e -> Key n

src/Vervis/Model.hs

@@ -56,6 +56,7 @@ instance Hashable ProjectRoleId where
     hash = hash . fromSqlKey
 
 instance PersistEntityGraph Ticket TicketDependency where
+    type PersistEntityGraphSelector Ticket TicketDependency = ProjectId
     sourceParam = ticketDependencyParent
     sourceField = TicketDependencyParent
     destParam   = ticketDependencyChild