push, pull, addToSet :: forall record typ.
        ( PersistField typ
        , PersistEntityBackend record ~ DB.MongoContext
        ) => EntityField record [typ] -> typ -> Update record
fld `push`     val = backendArrayOperation MongoPush     fld val
push, pull, addToSet :: forall record typ.
        ( PersistField typ
        , PersistEntityBackend record ~ DB.MongoContext
        ) => EntityField record [typ] -> typ -> Update record
fld `push`     val = backendArrayOperation MongoPush     fld val
fld `push`     val = backendArrayOperation MongoPush     fld val