用于Yesod / Persistent的MongoDB示例 [英] MongoDB Example for Yesod / Persistent

问题描述

Haskell和Yesod新手在这里。我一直在尝试遵循Yesod书（http://www.yesodweb.com/book/persistent）中Persistent章节中的Yesod示例的集成。 Sqlite编译并运行良好，似乎。但是，我试图使用MongDB，并且很难让事情顺利进行。特别是：

1. 在sqlite的示例中：
   
   

   share [mkPersist sqlSettings，mkMigratemigrateAll] [persist |

   
   
   

   Yesod书中说Mongo代码将使用 mongoSettings 代替。但是我无法在任何模块中找到它，并且代码不能编译。所以相反，我不得不使用它来代替 mongoSettings ：

   
   
   

   MkPersistSettings {mpsBackend = ConT''Action}

   
   
   

   我必须导入Language.Haskell.TH.Syntax才能编译它，我假设应该对用户隐藏，所以我当然不会这样做。

   
   
   

   另外，我发现在Mongo的持久测试中没有共享和迁移部分。我实际上不确定为什么，我猜这是因为MongoDB是无模式的，不需要迁移？




2. SqlPersist

   
   
   

   我认为 MongoPersist 会与SqlPersist相对应，几乎是 - 我在Persistent的test目录下的init.hs中找到了 MongoPersist 的一个实例。但它被评论了，所以我有一种感觉它已经过时了？无论如何，据我所知，它还没有定义。所以我不知道如何转换下面的行（Yesod书的P.115，或接近 http：// www.yesodweb.com/book/persistent ）为MongoDB工作：

     instance YesodPersist PersistTest其中
   类型YesodPersistBackend PersistTest = SqlPersist 
    runDB action = do 
    PersistTest池<  -  getYesod 
    runSqlPool动作池
     




3. 使用withMongoDBConn

   
   
   

   因此，对于sqlite，代码是（来自上面网页的第一个示例）：

   
   $ b $

     main :: IO（）
    main = withSqliteConn：memory：$ runSqlConn $ do 
    runMigration migrateAll 
    johnId<  - 插入$ PersonJohn Doe$只需35 
    ...等等
     

   
   
   但是
   
   

     main :: IO（）
    main = withMongoDBConn：memory：$ runMongoDBConn $ do 
    runMigration migrateAll 
    johnId<  -  inse rt $ PersonJohn Doe$只需35 
    ...等等
     

不起作用。首先，由于某种原因， runMigration 不在范围内。好的，也许我不需要迁移MongoDB，所以我删除了该行。然后，编译器会抱怨：
无法与预期类型匹配 AccessMode'与实际类型 m0 b0'
预期类型： m0 t0 - > （t 0 - > m 0 b 0） - > AccessMode
实际类型：m0 t0 - > （t 0 - > m 0 b 0） - > m0 b0 等等。在这个阶段，我对monads的粗略认识还不足以证明这一点。

总而言之，我在转换集成以及从Sqlite到MongoDB的书中的Yesod示例。有人能给我提供一个Yesod / Persistent with MongoDB的具体例子吗？非常感谢。

解决方案

我今天在Github Yesod Cookbook上添加了一个页面，它将MongoDB与Persistent结合使用。但是，它并不使用和MongoDBConn ，它也不能避免TH。另外，我解释了为什么我使用单独的YAML配置文件。链接： http://bit.ly/VLvmoK

Haskell and Yesod newbie here. I've been trying to follow the Integration with Yesod example from the Persistent chapter in the Yesod book (http://www.yesodweb.com/book/persistent). The Sqlite compiles and runs fine it seems. However, I'm trying to use MongDB and am having a hard time getting things to work. Specifically:

1. In the example for sqlite:

   share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persist|

   The Yesod book says "Mongo code will use mongoSettings instead." But I can't find it in any of the modules, and the code doesn't compile. So instead, I had to use this instead of mongoSettings:

   MkPersistSettings { mpsBackend = ConT ''Action }

   I had to import Language.Haskell.TH.Syntax to get it to compile, which I'm assuming should be hidden from the user so I'm certainly not doing it right.

   Also, I found that in the Persistent tests for Mongo do without the "share" and "migrate" part. I'm actually not sure why, I'm guessing it's because MongoDB is Schema-less and doesn't need migration?

2. SqlPersist

   I thought MongoPersist would be the counterpart to SqlPersist, and I guess it almost is--I found one instance of MongoPersist in the init.hs in the test directory of Persistent. But it's commented out, so I have a feeling it's obsolete? Otherwise it's not defined anyway as far as I can tell. So I don't know how to convert the following line (P.115 of the Yesod book, or near the end of http://www.yesodweb.com/book/persistent) to work for MongoDB:

   instance YesodPersist PersistTest where
       type YesodPersistBackend PersistTest = SqlPersist
       runDB action = do
           PersistTest pool <- getYesod
           runSqlPool action pool
   

3. Using withMongoDBConn

   So for sqlite, the code is (1st example from the webpage above):

   main :: IO ()
   main = withSqliteConn ":memory:" $ runSqlConn $ do
       runMigration migrateAll
       johnId <- insert $ Person "John Doe" $ Just 35
       ... and so on
   

   But

   main :: IO()
   main = withMongoDBConn ":memory:" $ runMongoDBConn $ do
       runMigration migrateAll 
       johnId <- insert $ Person "John Doe" $ Just 35
       ... and so on          
   

doesn't work. First off, runMigration for some reason isn't in scope. Okay, maybe I don't need migration for MongoDB, so I remove that line. Then, the compiler complains: Couldn't match expected typeAccessMode' with actual type m0 b0' Expected type: m0 t0 -> (t0 -> m0 b0) -> AccessMode Actual type: m0 t0 -> (t0 -> m0 b0) -> m0 b0 and so on. And by this stage my very cursory knowledge of monads just isn't enough to figure this one out.

All in all, I am having a really tough time converting the Integration with Yesod example from the book from Sqlite to MongoDB. Could someone please provide me with a concrete example of Yesod/Persistent with MongoDB? Much thanks in advance.

解决方案

I added a page in the Github Yesod Cookbook today that uses MongoDB in conjunction with Persistent. It does not, however, use withMongoDBConn, neither does it avoid TH. Also, I explain there why I use a separate YAML config file. The link: http://bit.ly/VLvmoK

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