在持久Yesod中进行多对多的最佳方法是什么？

Question

So my /config/models looks like this.

Person
  name Text
Car
  name Text
PersonCar
  personId PersionId eq
  carId CarId eq
  UniquePersonCar personId carId

Assume the inputs in the database are Person "Batman" Person "Superman" Car "SUV" Car "Ford" respectively.

I'm currently doing this to link them up in my Handler.

runDB $ do
  person <- selectFirst [PersonName ==. "Batman"] []
  car    <- selectFirst [Carname ==. "SUV"] []
  let Entity personId _ = case person of
                            Just info -> infor
                            Nothing -> error "no such Person"
  let Entity carId _ = case car of
                            Just info -> infor
                            Nothing -> error "no such Car"
  _ <- insert $ PersonCar personId carId

Is there an easier way to do this? Is there a convention for doing such expression?

Answer 1

不，目前没有这种查询的捷径（至少我能想到）。

Answer 2

calls to error will halt your app. logError may be better.

This is shorter:

import Data.Conduit
import qualified Data.Conduit.List as DCL

runDB $ do
    mbPersonId <- runResourceT $ selectKeys [PersonName ==. "Batman"] [] $$ DCL.head
    mbCarId    <- runResourceT $ selectKeys [CarName ==. "SUV"] [] $$ DCL.head

    case (mbPersonId, mbCarId) of
        (Just personId, Just carId) -> do
              _ <- insert $ PersonCar personId carId
              return ()

        _ -> $(logError) "error looking for Batman and SUV"