Linq在發票不包含任何項目時返回null

Question

我有一張發票，它可以包含稱爲軌道的項目。 InvoiceLine是連接兩者的聯結表。如果發票至少包含一首曲目，則一切正常。我想要實現的是即使不包含曲目，也要退回發票。

我最初的代碼：

var screenset = 
from inv in context.Invoices where inv.InvoiceId == invoiceID 
join line in context.InvoiceLines on inv.InvoiceId equals line.InvoiceId 
join track in context.Tracks on line.TrackId equals track.TrackId 
select new InvoiceAndItemsDTO 
{ 
    InvoiceId = inv.InvoiceId, 
    InvoiceDate = inv.InvoiceDate, 
    InvoiceTotal = inv.Total, 
    CustomerId = inv.CustomerId, 
    CustomerFullName = inv.Customer.LastName + ", " + inv.Customer.FirstName, 
    CustomerPhoneNumber = inv.Customer.Phone, 
    InvoiceLineId = line.InvoiceLineId, 
    TrackId = track.TrackId, 
    TrackName = track.Name, 
    TrackPrice = track.UnitPrice, 
    Artist = track.Album.Artist.Name, 
    UnitPrice = line.UnitPrice, 
    Quantity = line.Quantity, 
    Action = "None" 
}; 

以上將返回發票和正確的軌道，如果它包含任何軌道。

基本上上面的代碼是內連接，以便如果不存在磁道結果集將是空的。所以我糾正了我的代碼，即使它包含沒有軌道的發票也包括在內。

var screenset = 
from inv in context.Invoices where inv.InvoiceId == invoiceID 
from j in context.Invoices where j.InvoiceId == invoiceID 
join line in context.InvoiceLines on j.InvoiceId equals line.InvoiceId 
join track in context.Tracks on line.TrackId equals track.TrackId 
select new InvoiceAndItemsDTO 
{ 
    InvoiceId = inv.InvoiceId, 
    InvoiceDate = inv.InvoiceDate, 
    InvoiceTotal = inv.Total, 
    CustomerId = inv.CustomerId, 
    CustomerFullName = inv.Customer.LastName + ", " + inv.Customer.FirstName, 
    CustomerPhoneNumber = inv.Customer.Phone, 
    InvoiceLineId = line.InvoiceLineId, 
    TrackId = track.TrackId, 
    TrackName = track.Name, 
    TrackPrice = track.UnitPrice, 
    Artist = track.Album.Artist.Name, 
    UnitPrice = line.UnitPrice, 
    Quantity = line.Quantity, 
    Action = "None" 
}; 

，但我得到一個空的結果：

我嘗試使用join into newset和使用from e in newset.DefaultIfEmpty()但我加入3個表和join into生產的最後一個表並不能包括所有連接的表。

我欣賞這方面的任何幫助。

在此先感謝。

Answer 1

您可以使用左外部連接併爲軌道模型提供默認值。

Track defaultTrack= new Track() 
     { 
      TrackId = 0, 
      TrackName = "No Track", 
      TrackPrice = 0, 
      Artist = "No Artist", 
     }; 

//Left outer join 
var screenset = 
from inv in context.Invoices where inv.InvoiceId == invoiceID 
from j in context.Invoices where j.InvoiceId == invoiceID 
join line in context.InvoiceLines on j.InvoiceId equals line.InvoiceId 
join track in context.Tracks on line.TrackId equals track.TrackId into trackGroup 
from trackDetails in trackGroup.DefaultIfEmpty(defaultTrack) 
select new InvoiceAndItemsDTO 
{ 
    InvoiceId = inv.InvoiceId, 
    InvoiceDate = inv.InvoiceDate, 
    InvoiceTotal = inv.Total, 
    CustomerId = inv.CustomerId, 
    CustomerFullName = inv.Customer.LastName + ", " + inv.Customer.FirstName, 
    CustomerPhoneNumber = inv.Customer.Phone, 
    InvoiceLineId = line.InvoiceLineId, 
    TrackId = trackDetails .TrackId, 
    TrackName = trackDetails .Name, 
    TrackPrice = trackDetails .UnitPrice, 
    Artist = trackDetails .Album.Artist.Name, 
    UnitPrice = line.UnitPrice, 
    Quantity = line.Quantity, 
    Action = "None" 
}; 

Answer 2

很簡單。

讓乘坐原始查詢

from inv in context.Invoices where inv.InvoiceId == invoiceID 
join line in context.InvoiceLines on inv.InvoiceId equals line.InvoiceId 
join track in context.Tracks on line.TrackId equals track.TrackId 
... 

產生內部連接。爲了把它們變成一個左外連接，所有你需要的是插入兩行：

from inv in context.Invoices where inv.InvoiceId == invoiceID 
join line in context.InvoiceLines on inv.InvoiceId equals line.InvoiceId 
into lines from line in lines.DefaultIfEmpty() // turn the above to left join 
join track in context.Tracks on line.TrackId equals track.TrackId 
into tracks from track in tracks.DefaultIfEmpty() // turn the above to left join 
... 

見Left Outer Join的MSDN文檔。