Question

I have the following model:

User
  ...
Group
  ...
Sharing
  objectId (Either UserId GroupId)

In Sharing entity I want to store either UserId or GroupId and differentiate between them. Simply using Either doesn't work:

Not in scope: type constructor or class `UserId'

Not in scope: type constructor or class `GroupId'

Adding a new sum-type also doesn't work:

data SharingIdType = SharingUserId UserId | SharingGroupId GroupId

Not in scope: type constructor or class `SharingIdType'

Moving SharingIdType into another module isn't possible, because it uses UserId and GroupId types. The only way I see is to create an entity for each sharing type, like UserSharing/GroupSharing.

Other than that, how to approach this problem?

Answer 1

在搜索了一段时间并思考之后，我得出结论，有两种可能的解决方案：

1

如果SharingIdType的数量是静态的或很少更改（意味着可以重新编译源以更改它或更改数据库模式），处理问题的正确方法是必须使用实体每种分享类型：

User
  ...
Group
  ...
UserSharing
  userId UserId
GroupSharing
  groupId GroupId

这里＆＃34; sumness＆＃34;问题被转移到数据库查询。每当我需要找出共享的内容时，我会生成两个selectList并查询两个表而不是一个。

2

如果需要动态更改SharingIdTypes的数量，则需要SharingType实体：

User
  ...
Group
  ...
SharingType
  description String
Sharing
  objectId SharingTypeId

此表填充了与SharingIdType s构造函数对应的值：

do
  insert $ SharingType "user"
  insert $ SharingType "group"

现在每当我们分享某些内容时，我们都会引用SharingTypeId。

How do I store Either (Key a) (Key b)?

1 个答案:

1

2