Question

我有一个相当简单的查询，可以执行两个外连接。（一顿饭有很多食谱，反过来又有很多食物）。

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，它说明了它需要什么，没有更多。但是，由于SQL的工作方式，它为我提供了一个包含大量重复餐的表格，用于匹配的每个外部联接。

例如，一顿有两种食谱的食物，每种食物有两种食物，变成4个元组。

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

我想将这些备份转换为单个Meal数据类型。（这里简化为显示结构，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合并，并且最终成为此单个查询的2页左右的代码。

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

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)

提供各种复杂性的实例（代码：https://gist.github.com/cschneid/2989057ec4bb9875e2ae）

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

问题：

我想要公开的唯一内容是查询签名。剩下的就是实施垃圾。有没有使用Persistent的技巧我没有注意到？我是否必须手动将连接合并回haskell类型？

Answer 1

感谢@ JPMoresmau的暗示，我最终得到了一个更短的，我认为更简单的方法。由于nub，在大型数据集上可能会更慢，但在小型数据集上，它返回的速度远远超过我的需要。

我仍然讨厌我有太多的手动管道来构建从数据库返回的数据中的树结构。我想知道是否有一个很好的方法来做到这一点？

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

问题：

1 个答案: