Yesod：通过Int从ID获取数据库实体 [英] Yesod: Getting a database entity by ID from an Int

问题描述

我对Haskell和Yesod都很陌生，并且正在尝试构建一个简单的Web应用程序，它可以回答来自外部API的查询。我已经构建了一个解析器（使用Parsec），它可以获取我想要作为常规Int值加载的实体的ID。

然而，我为了我无法弄清楚如何将这个Int变成 get 会接受的东西（即 Key （？） ）。文档中的所有示例仅从前面的插入中获得id，或从url派发获得。

任何帮助都将不胜感激，因为我好像被卡住了... ：）

解决方案

即使答案已经可以在评论中找到，我想举一个完整的例子。假设我们有一个 Person Model，下面的函数返回给定ID的persion记录（如果存在的话） ）：

  import Database.Persist.Types（PersistValue（PersistInt64））
 
 getByIntId :: Integral i =>我 - > Handler（也许是人）
 getByIntId i = runDB $ get $ Key $ PersistInt64（fromIntegral i）
  

需要 import 来让我们构造一个整数的持久版本。 fromIntegral 将任何整数转换为期望的类型 Int64 。

更新：由于Yesod 1.2 PersistValue 存在于模块 Database.Persist.Types 中，在1.2之前它是 Database.Persist.Store （ API文档）。

更新2 ：从Persistent 2.0.2开始，有两个内置函数可以将数据库键转换为数据库键： toSqlKey 和 fromSqlKey code>（ API文档）。


I'm new to both Haskell and Yesod, and am trying to build a simple web application that can answer queries from an external API. I have built a parser (using Parsec), that gets me the ID of an entity I want to load as a regular Int value.

However, I for the life of me can't figure out how to turn this Int into something that get will accept (i. e. a Key (?)). All the examples in the documentation only get the id from previous inserts, or from url dispatch.

Any help would be greatly appreciated, since I seem to be stuck... :)

解决方案

Even if the answer can already be found in the comments, I would like to give a complete example.

Assuming we have a Person Model, the following function returns a record for the persion with the given ID (if it exists):

import Database.Persist.Types (PersistValue(PersistInt64))

getByIntId :: Integral i => i -> Handler (Maybe Person)
getByIntId i = runDB $ get $ Key $ PersistInt64 (fromIntegral i)

The import is needed to let us construct the persist-version of an integer. fromIntegral converts any integer to the expected type Int64.

Update: Since Yesod 1.2 PersistValue lives in the module Database.Persist.Types, before 1.2 it was Database.Persist.Store (API Documentation).

Update 2: Since Persistent 2.0.2 there are two build-in functions to convert from/to database keys: toSqlKey and fromSqlKey (API Documentation).

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