持久化中使用`get`和`toSqlKey` [英] Use of `get` and `toSqlKey` in persistent

问题描述

我有一个 User 模型。

我试图在服务器上使用persistent-postgresql。 / p>

我想要一个端点接受一个id并返回带有该id的用户。

根据其他答案我可以使用 toSqlKey 将 Int64 变成键 feed to get 。

我的功能如下所示：

  oneUser :: Int64  - > App（实体用户）
 oneUser userId = do 
 maybeUser<  -  runDb $ get $ toSqlKey userId 
 case mayUser of 
 Nothing  - > 
 throwError err404 
只是用户 - > 
返回用户
  

当我尝试编译时，出现错误无法匹配预期类型'PersistEntityBackend（实体用户）'与实际类型'SqlBackend'

使用 selectList

  allUsers :: App [实体用户] 
 allUsers = runDb $ selectList [] [] 
  

请告诉我我做错了什么，我应该在哪里看未来的东西是这样的。我找不到取得在hackage上/正确版本的图书馆在堆叠等。

runDb 看起来像：

  runDb ::（MonadReader Config m，MonadIO m）= > SqlPersistT IO b  - > mb 
 runDb query = do 
 pool<  - 请求getPool 
 liftIO $ runSqlPool查询池
  

取自这个github项目。

解决方案

区别在于 get ... 简单的用户不是实体用户，所以这将工作：

  altSingleUser :: Int64  - >应用用户
 altSingleUser userid = do 
 let foo = get（toSqlKey userid）:: SqlPersistT IO（也许用户）
 maybeUser<  -  runDb $ foo 
 case可能用户
 Nothing  - > 
 throwError err404 
只是人 - > 
 return person 
  

如果您要返回实体用户，只需将最后一条return语句改为：

  return $实体{entityKey = toSqlKey userid，entityVal = person} 
  

I'm trying to use persistent-postgresql with servant.

I have a User model.

I want to have an endpoint that takes an id and returns the user with that id.

According to other answers I can use toSqlKey to turn an Int64 into a Key to feed to get.

My function looks like:

oneUser :: Int64 -> App (Entity User)
oneUser userId = do
  maybeUser <- runDb $ get $ toSqlKey userId
  case maybeUser of
    Nothing ->
      throwError err404
    Just user ->
      return user

When I try to compile I get the error Couldn't match expected type ‘PersistEntityBackend (Entity User)’ with actual type ‘SqlBackend’

Use of selectList works fine.

allUsers :: App [Entity User]
allUsers = runDb $ selectList [] []

Please tell me what I'm doing wrong and where I should look in the future for stuff like this. I couldn't find get on hackage/the right version of the library on stackage etc.

runDb looks like:

runDb :: (MonadReader Config m, MonadIO m) => SqlPersistT IO b -> m b
runDb query = do
  pool <- asks getPool
  liftIO $ runSqlPool query pool

taken from this github project.

解决方案

The difference is that get ... returns a plain User not an Entity User, so this will work:

altSingleUser :: Int64 -> App User
altSingleUser userid = do
    let foo = get (toSqlKey userid) :: SqlPersistT IO (Maybe User)
    maybeUser <- runDb $ foo
    case maybeUser of
         Nothing ->
            throwError err404
         Just person ->
            return person

If you want to return an Entity User, just change the last return statement to:

return $ Entity { entityKey = toSqlKey userid, entityVal = person }

这篇关于持久化中使用`get`和`toSqlKey`的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！