[英]Haskell Esqueleto - Maybe Keys don't bind on LeftOuterJoin
I am with some trouble while making a join which uses a maybe foreign key
. 在使用
maybe foreign key
的maybe foreign key
时,我遇到了一些麻烦。
My Models: 我的模特:
OrderItem
productSnapshot ProductSnapshotId
ProductFlow
productInstance ProductInstanceId
orderItem OrderItemId Maybe
ProductSnapshot
productInstance ProductInstanceId
ProductInstance
code Text
And my joins: 和我的加入:
type Entities = [(Entity OrderItem, Entity ProductSnapshot, Entity ProductInstance, Maybe (Entity ProductFlow))]
loadEntities :: OrderItemId -> Handler Entities
loadEntities oiId = runDB
$ E.select
$ E.from $ \(oi `E.InnerJoin` ps `E.InnerJoin` pi `E.LeftOuterJoin` pf) -> do
E.on $ pf E.?. ProductFlowOrderItem E.==. E.just (oi ^. OrderItemId)
E.on $ pi ^. ProductInstanceId E.==. ps ^. ProductSnapshotProductInstance
E.on $ ps ^. ProductSnapshotId E.==. oi ^. OrderItemProductSnapshot
E.where_ (oi ^. OrderItemId E.==. E.val oiId)
return (oi, ps, pi, pf)
I tried the Esqueleto
join showed above, but I got the following error: 我尝试了
Esqueleto
显示的Esqueleto
连接,但是出现以下错误:
Couldn't match type ‘Key OrderItem’ with ‘Maybe (Key OrderItem)’
Expected type: EntityField OrderItem (Maybe (Key OrderItem))
Actual type: EntityField OrderItem (Key OrderItem)
In the second argument of ‘(^.)’, namely ‘OrderItemId’
In the first argument of ‘E.just’, namely ‘(oi ^. OrderItemId)’
I think it is caused because I'm using a maybe foreign key. 我认为这是因为我使用的是外键。 I tried other ways, and got strange type errors.
我尝试了其他方法,但遇到了奇怪的类型错误。 I think I don't understand completely well how these joins work, so a brief explanation about
E.^.
我想我不太了解这些连接的工作原理,因此请简要介绍一下
E.^.
, E.just
and E.?.
,
E.just
和E.?.
would improve my Esqueleto usage. 会提高我的Esqueleto使用率。
I managed to understand the problem and solve it myself, instead of 我设法了解了问题并自己解决了,而不是
E.on $ pf E.?. ProductFlowOrderItem E.==. E.just (oi ^. OrderItemId)
use this 用这个
E.on $ pf E.?. ProductFlowOrderItem E.==. (E.just . E.just) (oi ^. OrderItemId)
It is necessary to make the types match, because we are using E.?.
必须使类型匹配,因为我们使用的是
E.?.
with an already Maybe
value, so, we have to make the right side a Maybe (Maybe AType)
, otherwise it would be wrong, and to make it I just used E.just . E.just
使用已经存在的
Maybe
值,因此,我们必须将右侧Maybe (Maybe AType)
,否则将是错误的,并且我只使用E.just . E.just
E.just . E.just
which will wrap two times. E.just . E.just
将包装两次。
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