简化持久性和 Esqueleto 代码

Question

我有一个相当简单的查询，它执行两个外部联接。（一顿饭有很多食谱，而食谱又有很多食物）。

getMeals :: (MonadIO m) => Key DbUser -> SqlPersistT m [Meal]
getMeals user =
  fmap deserializeDb $ E.select $
        E.from $ \(m `E.InnerJoin` u `E.LeftOuterJoin` r `E.LeftOuterJoin` f) -> do
          E.on     (r ?. DbRecipeId E.==. f ?. DbFoodRecipeId)
          E.on     (E.just (m ^. DbMealId) E.==. r ?. DbRecipeMealId)
          E.on     (m ^. DbMealUserId      E.==. u ^. DbUserId)
          E.where_ (m ^. DbMealUserId      E.==. E.val user )
          return (m, r, f)

这个查询很棒，它只说了它所需要的，仅此而已。但是，由于 SQL 的工作原理，对于每个匹配的外部连接，它会返回一个 table 和很多重复的饭菜。

比如一顿饭有两个食谱，每个食谱有两种食物，变成4个元组。

(m1, r1, f1)
(m1, r1, f2)
(m1, r2, f3)
(m1, r2, f4)

我想将它们回滚到一个 Meal 数据类型中。（此处简化显示结构，其他字段当然存储在DB中）。

data Meal   = Meal   { recipes :: [Recipe] }
data Recipe = Recipe { foods :: [Food]   }
data Food   = Food   { name :: String }

我似乎必须完全手动进行此合并，最终这个单个查询需要 2 页左右的代码。

忽略类型类不应该像这样使用的事实，它看起来像一个（愚蠢的）类型类的很多实例 DeserializeDb:

class DeserializeDb a r | a -> r where
  deserializeDb :: a -> r

instance DeserializeDb [(Entity DbMeal, Maybe (Entity DbRecipe))] [Meal] where
  deserializeDb items = let grouped = groupBy (\a b -> entityKey (fst a) == entityKey (fst b)) items
                            joined  = map (\list -> ( (fst . head) list
                                                    ,  mapMaybe snd list
                                                    )) grouped
                        in (map deserializeDb joined)

截取了大量不同复杂度的实例（代码：https://gist.github.com/cschneid/2989057ec4bb9875e2ae）

instance DeserializeDb (Entity DbFood) Food where
  deserializeDb (Entity _ val) = Food (dbFoodName val)

问题：

我唯一想公开的是查询签名。其余部分是实现垃圾。是否有我没有注意到的使用 Persistent 的技巧？我是否必须手动将联接合并回 haskell 类型？

Answer 1

感谢@JPMoresmau 的暗示，我最终得到了一个更短的方法，而且我认为更简单的方法。由于 nub，它在大型数据集上可能会更慢，但在小型数据集上它 returns 比我需要的要快得多。

我仍然讨厌我有这么多手动管道来根据从数据库返回的数据构建树结构。请问有没有什么好的方法可以通用？

module Grocery.Database.Calendar where

import Grocery.DatabaseSchema
import Grocery.Types.Meal
import Grocery.Types.Recipe
import Grocery.Types.Food
import Database.Persist
import Database.Persist.Sqlite
import qualified Database.Esqueleto      as E
import           Database.Esqueleto      ((^.), (?.))
import Data.Time
import Control.Monad.Trans -- for MonadIO
import Data.List
import Data.Maybe
import Data.Tuple3

getMeals :: (MonadIO m) => Key DbUser -> SqlPersistT m [Meal]
getMeals user =
  fmap deserializeDb $ E.select $
        E.from $ \(m `E.InnerJoin` u `E.LeftOuterJoin` r `E.LeftOuterJoin` f) -> do
          E.on     (r ?. DbRecipeId E.==. f ?. DbFoodRecipeId)
          E.on     (E.just (m ^. DbMealId) E.==. r ?. DbRecipeMealId)
          E.on     (m ^. DbMealUserId      E.==. u ^. DbUserId)
          E.where_ (m ^. DbMealUserId      E.==. E.val user )
          return (m, r, f)

deserializeDb :: [(Entity DbMeal, Maybe (Entity DbRecipe), Maybe (Entity DbFood))] -> [Meal]
deserializeDb results = makeMeals results
  where
    makeMeals :: [(Entity DbMeal, Maybe (Entity DbRecipe), Maybe (Entity DbFood))] -> [Meal]
    makeMeals dupedMeals = map makeMeal (nub $ map fst3 dupedMeals)

    makeMeal :: Entity DbMeal -> Meal
    makeMeal (Entity k m) = let d = dbMealDay m
                                n = dbMealName m
                                r = makeRecipesForMeal k
                            in  Meal Nothing (utctDay d) n r

    makeRecipesForMeal :: Key DbMeal -> [Recipe]
    makeRecipesForMeal mealKey = map makeRecipe $ appropriateRecipes mealKey

    appropriateRecipes :: Key DbMeal -> [Entity DbRecipe]
    appropriateRecipes mealKey = nub $ filter (\(Entity _ v) -> dbRecipeMealId v == mealKey) $ mapMaybe snd3 results

    makeRecipe :: Entity DbRecipe -> Recipe
    makeRecipe (Entity k r) = let n = dbRecipeName r
                                  f = makeFoodForRecipe k
                              in  Recipe Nothing n f

    makeFoodForRecipe :: Key DbRecipe -> [Food]
    makeFoodForRecipe rKey = map makeFood $ appropriateFoods rKey

    appropriateFoods :: Key DbRecipe -> [Entity DbFood]
    appropriateFoods rKey = nub $ filter (\(Entity _ v) -> dbFoodRecipeId v == rKey) $ mapMaybe thd3 results

    makeFood :: Entity DbFood -> Food
    makeFood (Entity _ f) = Food (dbFoodName f)

简化持久性和 Esqueleto 代码

Simplifying Persistent & Esqueleto code

haskell

yesod

persistent

esqueleto

问题：