在LINQ WHERE子句中有条件地允许null

Question

工作：

我正在尝试在Contacts表和Permissions表之间执行LEFT OUTER JOIN。我有正确的工作基础，并返回一个联系人列表，无论他们是否有相应的权限。

// 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WHERE SQL：

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

问题：

我想调整以上内容来介绍'后备'检查;没有按预期工作。

要求：

• 如果Permission没有Contact（即p==null），则只根据bool的预定allowed值包含该行。

尝试：

我以为我可以这样做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：

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）

时

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

实际结果：

即使在allowed=false？

时，始终像true一样输出

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

要点：

我希望我只是做一些容易修复的傻事 如何根据给定的null值过滤我的bool值记录？

Answer 1

您在此处执行GroupJoin。因此，第一部分permissionGrp的结果是匿名类型IGrouping<Permission>。这已经是OUTER JOIN的等价物。

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

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 };

请注意，from p in permissionGrp会再次展平分组，因此.DefaultIfEmpty()的情况仍然需要allowed == true。

Answer 2

我怀疑我正在使用的ORM（LightSpeed）中存在一个错误，我会引起他们的注意。

解决方法

我找到了一个合适的工作，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

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

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

时的情况

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

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

时的情况

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