在 LINQ WHERE 子句中有条件地允许 null

Question

工作：

我正在尝试在我的 Contacts 表和我的 Permissions 表之间执行 LEFT OUTER JOIN。我有这个正常工作的基础，无论他们是否有相应的权限，都可以取回一个联系人列表。

// Query for Contacts
from contact in Contacts
join permission in Permissions on contact.Id equals permission.ObjectId into permissionGrp
from p in permissionGrp.DefaultIfEmpty()            
where p==null || (p!=null && /* ... condition based on the existence of a permission */)
select new { contact, permission = p };

生成的 WHERE SQL：

WHERE
(t1.PermissionId IS NULL OR 
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... )))

问题：

我想修改上述内容以引入“后备”检查；没有按预期工作。

要求：

• 当没有Contact 对应的Permission 时（即p==null），则仅包含基于预定bool 值allowed 的行。

尝试：

我以为我可以这样where (p==null && allowed) || ...：

// Fallback permission
bool allowed = false;

// Query for Contacts
from contact in Contacts
join permission in Permissions on contact.Id equals permission.ObjectId into permissionGrp
from p in permissionGrp.DefaultIfEmpty()

/* Added bool condition 'allowed' to control inclusion when 'p' is null */
where (p==null && allowed) || (p!=null && /* ... condition based on the existence of a permission */)
select new { contact, permission = p };

预期：

当allowed = false（不接受null权限）

WHERE
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... ))

当allowed = true（接受null权限）

WHERE
(t1.PermissionId IS NULL OR 
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... )))

实际结果：

即使true 也总是像allowed=false 一样输出？

WHERE
    ((t1.PermissionId IS NOT NULL AND ... other conditions ... ))

总结：

我希望我只是在做一些很容易解决的傻事。
如何根据给定的bool 值过滤我的null 值记录？

Answer 1

你在这里执行GroupJoin。所以第一部分的结果，permissionGrp，是一个匿名类型IGrouping<Permission>。这已经相当于 OUTER JOIN。

您可以通过有条件地测试IGrouping<Permission> 是否包含元素来实现您想要的：

from contact in Contacts
join permission in Permissions on contact.Id equals permission.ObjectId
    into permissionGrp
where allowed || g.Any()
from p in permissionGrp.DefaultIfEmpty()
select new { contact, permission = p };

注意from p in permissionGrp 再次展平了分组，因此.DefaultIfEmpty() 对于allowed == true 的情况仍然是必需的。

Answer 2

我怀疑我正在使用的 ORM (LightSpeed) 中存在错误，我会提请他们注意。

解决方法

我找到了一个合适的解决方法，使用let 子句。

// Fallback permission
bool allowed = false;

// Query for Contacts
from contact in Contacts
join permission in Permissions on contact.Id equals permission.ObjectId into permissionGrp
from p in permissionGrp.DefaultIfEmpty()

/* Work around for 'allowed' not being honoured properly, using 'let' */
let isAllowed = allowed

/* Added bool condition 'isAllowed' to control inclusion when 'p' is null */
where (p==null && isAllowed) || (p!=null && /* ... condition based on the existence of a permission */)
select new { contact, permission = p };

结果

它现在使用已知值与自身的比较作为布尔检查。在这种情况下t0.ContactId。

SQL 当allowed=true ... t0.ContactId = t0.ContactId

WHERE
    ((t1.PermissionId IS NULL AND t0.ContactId = t0.ContactId) OR 
     (t1.PermissionId IS NOT NULL AND ... other conditions ...))

SQL 当allowed=false ... t0.ContactId <> t0.ContactId

WHERE
    ((t1.PermissionId IS NULL AND t0.ContactId <> t0.ContactId) OR 
     (t1.PermissionId IS NOT NULL AND ... other conditions ...))