Haskell Yesod：尝试打印表数据并从另一个表获取外键信息

我正在与Yesod合作，我正在尝试打印一个包含事件列表和它们所属城市的表格

我有以下型号：

City
    name Text
    countryId CountryId
    UniqueCity name
    deriving Eq
    deriving Show

Event
    title Text
    description Text
    date UTCTime
    cityId CityId
    deriving Eq
    deriving Show`

以及处理程序中的以下代码：

`module Handler.Event where

import Import
import Yesod.Form.Bootstrap3
import Database.Persist.Sql

getEventR :: Handler Html
getEventR = do
    events <- runDB $ selectList [] [Asc EventTitle]
    defaultLayout $ do 
        setTitle "Events"   
        [whamlet|
            <h1>Events
                <table>
                    <thead>
                        <tr>
                          <th>Event title</th>
                          <th>Event date</th>
                          <th>Event city</th>
                    <tbody>
                    $forall Entity eventId event <- events
                        <tr>
                            <td>#{eventTitle event}
                            <td>#{show (eventDate event)}   
                            <td>#{addCity eventId}   
        |]

addCity event = do
    city <- getCity $ eventCityId event
    return city

getCity cid = do
    city <- runDB $ get404 cid
    return city`

module Handler.Event where

import Import
import Yesod.Form.Bootstrap3
import Database.Persist.Sql

getEventR :: Handler Html
getEventR = do
    events <- runDB $ selectList [] [Asc EventTitle]
    defaultLayout $ do 
        setTitle "Events"   
        [whamlet|
            <h1>Events
                <table>
                    <thead>
                        <tr>
                          <th>Event name</th>
                          <th>Event date</th>
                          <th>Event city</th>
                    <ul>
                    <tbody>
                    $forall ev <- map entityVal events
                        <tr>
                            <td>#{eventTitle ev}
                            <td>#{show (eventDate ev)}   
                            <td>#{addCity ev}   
        |]

addCity ev = do
    eve <- entityVal ev
    city <- getCity $ eventCityId eve
    return city

getCity cid = do
    city <- runDB $ get404 cid
    return $ cityName city

Couldn't match expected type 'Entity (HandlerT site0 IO Event)' with actual type 'Event'
In the first argument of 'addCity', namely 'ev_ajRc'
In the first argument of 'toHtml', namely 'addCity ev_ajRc'