使用ADO.NET删除数据库条目

Question

我有以下代码连接到我的数据库，并检查电子邮件地址是否存在于表中，如果存在，它将删除它，如果没有，它将添加它。

但这两个选项都不起作用。我得到了两个动作相同的错误：

对象不能被删除，因为它没有在 ObjectStateManager找到。

代码：

protected OEntities database = new OEntities(); 

public string NewsletterSubscriptionValidation(string emailAddress) 
{ 
    string validationMessage = ""; 
    Newsletter newsletter = new Newsletter(); 

    var identifier = database.Newsletters.Select(
     id => id.EmailAddress); 

    if (identifier == null) 
    { 
     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = true; 

     database.Newsletters.AddObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You are now subscribed to our periodic Newsletter."; 
    } 
    else 
    { 
     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = false; 


     database.DeleteObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You have been unsubscribed from our periodic Newsletter."; 
    } 

    return validationMessage; 
} 

难道我甚至增加/删除数据库中的正确方法，在这里？

解决方案

---

protected OEntities database = new OEntities(); 

public string NewsletterSubscriptionValidation(string emailAddress) 
{ 
    string validationMessage = ""; 
    Newsletter newsletter = database.Newsletters.FirstOrDefault(
     nl => nl.EmailAddress == emailAddress); 

    if (newsletter == null) 
    { 
     newsletter = new Newsletter(); 

     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = true; 

     database.AddToNewsletters(newsletter); 
     //database.Newsletters.AddObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You are now subscribed to our periodic Newsletter."; 
    } 
    else 
    { 
     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = true; 

     database.AddToNewsletters(newsletter); 
     //database.Newsletters.AddObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You have been unsubscribed from our periodi Newsletter."; 
    } 

    return validationMessage; 
} 

Answer 1

您需要删除

database.Attach(newsletter); 
    database.DeleteObject(newsletter); 
    database.SaveChanges(); 

之前附加，但你可能想选择现有的资料更多类似这样

Newsletter newsletter = database.Newsletters.FirstOrDefault(nl => nl.EmailAddress == emailAddress); 
if (newsletter == null) 
{ 
    newsletter = new Newsletter(); 
    ... 
} 
else 
{ 
    database.Attach(newsletter); 
    database.DeleteObject(newsletter); 
    database.SaveChanges(); 
}