在LINQ WHERE子句中有條件地允許null

Question

工作：

我試圖在我的Contacts表和我的Permissions表之間執行LEFT OUTER JOIN。 我有正確的工作基礎，並獲取聯系人列表，無論他們是否有相應的權限。

// 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 ... )))

問題：

我想改編以上內容來介紹'后備'檢查; 沒有按預期工作。

需求：

• 當沒有對應於Contact Permission （即p==null ）時，則僅包括基於allowed的預定bool值的行。

嘗試：

我以為我可以where (p==null && allowed) || ... where (p==null && allowed) || ...像這樣：

// 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 };

預期：

當allowed = false （不接受null權限）

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

當allowed = true （接受null權限）

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

實際結果：

即使是true總是輸出，如果allowed=false ？

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

摘要：

我希望我只是做一些很容易修復的傻事。
如何根據給定的bool值過濾我的null值記錄？

Answer 1

您正在此處執行GroupJoin 。 因此第一部分permissionGrp的結果是一個匿名類型IGrouping<Permission> 。 這已經是OUTER JOIN的等價物了。

您可以通過有條件地測試IGrouping<Permission>包含元素來實現您的目的：

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 };

請注意， from p in permissionGrp再次展平分組，因此對於allowed == true的情況，仍然需要.DefaultIfEmpty() 。

Answer 2

我懷疑我正在使用的ORM（ LightSpeed ）中有一個錯誤，我會引起他們的注意。

解決方法

我使用let子句找到了合適的解決方法。

// 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 };

結果

它現在使用已知值與其自身的比較作為布爾檢查。 在這種情況下t0.ContactId 。

SQL當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當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 ...))