將實體映射到DTO

Question

我有一個EF實體，我想使用流暢的語法將其屬性映射到具有函數的DTO。

就拿用戶我能使其工作映射這樣說：

public Task<List<JournalTransactionModel>> GetAllJournalRecords() 
    { 
     var journalRecords = db.JournalTransactions 
      .Include(_ => _.JournalTransactionsAccounts) 
      .Include(_ => _.User) 
      .Select(_ => new JournalTransactionModel 
      { 
       JournalTransactionId = _.JournalTransactionId, 
       Date = _.Date, 
       Description = _.Description, 
       User = new UserModel 
       { 
        UserId = _.UserId, 
        FirstName = _.User.FirstName, 
        LastName = _.User.LastName, 
        FullName = _.User.FirstName + " " + _.User.LastName, 
        Email = _.User.Email, 
        UserName = _.User.UserName, 
        Password = _.User.Password, 
        UserRoleAndPermissions = new UserRoleModel 
        { 
         UserRoleId = _.User.UserRole.UserRoleId, 
         UserRoleName = _.User.UserRole.UserRoleName, 
         CanRead = _.User.UserRole.CanRead, 
         CanWrite = _.User.UserRole.CanWrite 
        } 
       }, 
       TransactionAccounts = _.JournalTransactionsAccounts.Select(j => new JournalTransactionAccountModel 
       { 
        JournalTransactionAccountId = j.Id, 
        JournalTransactionId = j.JournalTransactionId, 
        DebitAccount = j.DebitAccount != null ? new AccountModel 
        { 
         AccountId = j.DebitAccount.AccountId, 
         AccountCategoryName = j.DebitAccount.AccountCategory.AccountCategoryName, 
         AccountCategoryId = j.DebitAccount.AccountCategoryId, 
         AccountName = j.DebitAccount.AccountName, 
         IncreasesWhenDebited = j.DebitAccount.IncreasesWhenDebited 
        } : null, 
        CreditAccount = j.CreditAccount != null ? new AccountModel 
        { 
         AccountId = j.CreditAccount.AccountId, 
         AccountCategoryName = j.CreditAccount.AccountCategory.AccountCategoryName, 
         AccountCategoryId = j.CreditAccount.AccountCategoryId, 
         AccountName = j.CreditAccount.AccountName, 
         IncreasesWhenDebited = j.CreditAccount.IncreasesWhenDebited 
        } : null, 
        Amount = j.Amount, 
        Person = j.Person != null ? new PersonModel 
        { 
         PersonId = j.PersonId, 
         FirstName = j.Person.FirstName, 
         LastName = j.Person.LastName, 
         FullName = j.Person.FirstName + " " + j.Person.LastName, 
         Email = j.Person.Email, 
         SocialSecurityNumber = j.Person.SocialSecurityNumber, 
         PersonType = new PersonTypeModel 
         { 
          Id = j.Person.PeopleType.PeopleTypeId, 
          Name = j.Person.PeopleType.Name 
         } 
        } : null 
       }).ToList() 
      }).ToListAsync(); 
     return journalRecords; 
    } 

但是，當我試圖創建一個返回的usermodel我一直得到例外

public Task<List<JournalTransactionModel>> GetAllJournalRecords() 
    { 
     var journalRecords = db.JournalTransactions 
      .Include(_ => _.JournalTransactionsAccounts) 
      .Include(_ => _.User) 
      .Select(_ => new JournalTransactionModel 
      { 
       JournalTransactionId = _.JournalTransactionId, 
       Date = _.Date, 
       Description = _.Description, 
       User = MapUserToModel(_.User), 
       TransactionAccounts = _.JournalTransactionsAccounts.Select(j => new JournalTransactionAccountModel 
       { 
        JournalTransactionAccountId = j.Id, 
        JournalTransactionId = j.JournalTransactionId, 
        DebitAccount = j.DebitAccount != null ? new AccountModel 
        { 
         AccountId = j.DebitAccount.AccountId, 
         AccountCategoryName = j.DebitAccount.AccountCategory.AccountCategoryName, 
         AccountCategoryId = j.DebitAccount.AccountCategoryId, 
         AccountName = j.DebitAccount.AccountName, 
         IncreasesWhenDebited = j.DebitAccount.IncreasesWhenDebited 
        } : null, 
        CreditAccount = j.CreditAccount != null ? new AccountModel 
        { 
         AccountId = j.CreditAccount.AccountId, 
         AccountCategoryName = j.CreditAccount.AccountCategory.AccountCategoryName, 
         AccountCategoryId = j.CreditAccount.AccountCategoryId, 
         AccountName = j.CreditAccount.AccountName, 
         IncreasesWhenDebited = j.CreditAccount.IncreasesWhenDebited 
        } : null, 
        Amount = j.Amount, 
        Person = j.Person != null ? new PersonModel 
        { 
         PersonId = j.PersonId, 
         FirstName = j.Person.FirstName, 
         LastName = j.Person.LastName, 
         FullName = j.Person.FirstName + " " + j.Person.LastName, 
         Email = j.Person.Email, 
         SocialSecurityNumber = j.Person.SocialSecurityNumber, 
         PersonType = new PersonTypeModel 
         { 
          Id = j.Person.PeopleType.PeopleTypeId, 
          Name = j.Person.PeopleType.Name 
         } 
        } : null 
       }).ToList() 
      }).ToListAsync(); 
     return journalRecords; 

功能那裏有什麼不對？

這是我得到的消息：

「ExceptionMessage」：「LINQ到實體無法識別方法「ACS.Hub.BusinessLogic.Models.UserModel MapUserToModel（ACS.Hub.Repository.User） '方法，並且此方法不能轉換爲商店表達式。「

Answer 1

LINQ在服務器上查詢表達式翻譯IQueryable查詢。在你的情況下不支持，因爲你的子查詢必須在內存上計算。 MapUserToModel方法想要使用計算的數據。如果要通過MapUserToModel方法填充用戶索引，則可以在Select數據之前使用ToList()獲取內存數據。

var journalRecords = db.JournalTransactions 
      .Include(_ => _.JournalTransactionsAccounts) 
      .Include(_ => _.User) 
      .ToList() 
      .Select(_ => new JournalTransactionModel 
      { 
       .... 
      } 

Answer 2

你不能在linq查詢中調用你自定義的方法。原因在於，在後臺LINQ將所有LINQ語句轉換爲有效的SQL語句。而SQL語言當然沒有一個名爲MapUserToModel的函數的定義。一種解決方法是從您的MapUserToModel方法中取出代碼，並將其直接放入LINQ查詢中。

---

編輯： 我們已經討論過，爲什麼你不能使用查詢中的自定義方法，我們希望在SQL服務器上執行，我們也討論了，這是不是一件好事使用ToList（）並在內存中執行映射。但是，無論何時想要映射用戶，您都仍然面臨着編寫冗餘代碼的問題。由於我非常討厭看/看冗餘代碼，所以我花了一些時間來處理這種挑戰，並找到了一個解決方案，它可以讓您只在一個地方寫映射代碼，並在需要的地方使用它。因此，假設我們保留在數據庫中的原始用戶實體名爲User，我們的DTO類名爲UserModel。下面是代碼：

public class UserModel 
{ 
    public string Username { get; set; } 
    public string Email { get; set; } 
    public DateTime Birthday { get; set; } 

    public DbUser DbUser 
    { 
     set 
     { 
      Username = value.UserName; 
      Email = value.Email; 
      Birthday = value.Birthday; 
     } 
    } 
} 

這裏是我們如何使用：

var journalRecords = db.JournalTransactions 
.Include(_ => _.JournalTransactionsAccounts) 
.Include(_ => _.User) 
.Select(_ => new JournalTransactionModel 
{ 
    JournalTransactionId = _.JournalTransactionId, 
    Date = _.Date, 
    Description = _.Description, 
    User = new UserModel 
    { 
     DbUser = _ 
    } 
    ... 
}); 

我覺得代碼是自我解釋，但總之，這裏的技巧是，在我們的DTO的usermodel類我們爲原始DbUser實體添加一個屬性，我們只創建一個setter方法，它訪問DbUser中的每個屬性並將其分配給它自己的每個相應屬性。因此，如果您將來需要爲映射添加或刪除一些屬性，則只需轉到此一個setter方法並在此處實施更改。

Answer 3

您需要調用Select()之前調用toList()，但是這可能是嚴重降低查詢的性能。這就是爲什麼你會得到一個錯誤，因爲Linjan不知道如何將你的自定義方法翻譯成SQL，就像Dejanin說的那樣。但是，如果調用toList()方法，則會執行先前的查詢，而不再使用SQL完成操作。