本文將演示如何通過 Entity Framework 數(shù)據(jù)模型創(chuàng)建、修改、刪除數(shù)據(jù)庫記錄。
為了在數(shù)據(jù)庫里創(chuàng)建新紀錄，需要創(chuàng)建相應(yīng)實體類的新實例，填充字段，把實體類加入 ObjectContext 派生類維護的 EntityCollection，然后調(diào)用 SaveChanges（）寫入新紀錄：
Customer cust = new Customer(){ CustomerID = "LAWN", CompanyName = "Lawn Wranglers", ContactName = "Mr. Abe Henry", ContactTitle = "Owner", Address = "1017 Maple Leaf Way", City = "Ft. Worth", Region = "TX", PostalCode = "76104", Country = "USA", Phone = "(800) MOW-LAWN", Fax = "(800) MOW-LAWO"};
NorthwindEntities db = new NorthwindEntities();db.Customers.AddObject(cust);
db.SaveChanges();
可以多次 AddObject（）后調(diào)用一次 SaveChanges（） 全部寫入數(shù)據(jù)庫。
1. 創(chuàng)建部分加載的實體類
在之前的示例里，我們調(diào)用了 Customer 實體類的默認構(gòu)造函數(shù)，它創(chuàng)建的實例沒有加載任何數(shù)據(jù)。不過，我們還可以通過在構(gòu)造函數(shù)里指定必須字段的值來減少數(shù)據(jù)庫錯誤的風(fēng)險。
每個實體類都有一個名為 CreateT 的工廠方法，例如，Customer 的實體類的工廠方法是 CreateCustomer 。看下面示例：
Customer cust = Customer.CreateCustomer("LAWN", "Lawn Wranglers");
cust.ContactName = "Mr. Abe Henry";cust.ContactTitle = "Owner";cust.Address = "1017 Maple Leaf Way";cust.City = "Ft. Worth";cust.Region = "TX";cust.PostalCode = "76104";cust.Country = "USA";cust.Phone = "(800) MOW-LAWN";cust.Fax = "(800) MOW-LAWO";NorthwindEntities db = new NorthwindEntities();db.Customers.AddObject(cust);
db.SaveChanges();
我們傾向于使用默認構(gòu)造函數(shù)，因為可以在一條語句里指定屬性的值，但是如果你經(jīng)常會忘記給必需的字段賦值，那么工廠方法對你就非常有用。
2. 插入關(guān)聯(lián)的實體
可以用實體類的導(dǎo)航屬性創(chuàng)建一組關(guān)聯(lián)的對象，然后一次把它們存儲到數(shù)據(jù)庫：
Customer cust = new Customer{ CustomerID = "LAWN", CompanyName = "Lawn Wranglers", ContactName = "Mr. Abe Henry", ContactTitle = "Owner", Address = "1017 Maple Leaf Way", City = "Ft. Worth", Region = "TX", PostalCode = "76104", Country = "USA", Phone = "(800) MOW-LAWN", Fax = "(800) MOW-LAWO", Orders = { new Order{ CustomerID = "LAWN",EmployeeID = 4,
OrderDate = DateTime.Now,
RequiredDate = DateTime.Now.AddDays(7),
ShipVia = 3,
Freight = new Decimal(24.66), ShipName = "Lawn Wranglers", ShipAddress = "1017 Maple Leaf Way", ShipCity = "Ft. Worth", ShipRegion = "TX", ShipPostalCode = "76104", ShipCountry = "USA" }
}
};
NorthwindEntities db = new NorthwindEntities();db.Customers.AddObject(cust);
db.SaveChanges();
如果單獨創(chuàng)建 Order 和 Customer 對象，就不得不顯式的添加 Order：
Customer cust = new Customer{ CustomerID = "LAWN", CompanyName = "Lawn Wranglers", ContactName = "Mr. Abe Henry", ContactTitle = "Owner", Address = "1017 Maple Leaf Way", City = "Ft. Worth", Region = "TX", PostalCode = "76104", Country = "USA", Phone = "(800) MOW-LAWN", Fax = "(800) MOW-LAWO",};
Order ord = new Order{ CustomerID = "LAWN",EmployeeID = 4,
OrderDate = DateTime.Now,
RequiredDate = DateTime.Now.AddDays(7),
ShipVia = 3,
Freight = new Decimal(24.66), ShipName = "Lawn Wranglers", ShipAddress = "1017 Maple Leaf Way", ShipCity = "Ft. Worth", ShipRegion = "TX", ShipPostalCode = "76104", ShipCountry = "USA"};
NorthwindEntities db = new NorthwindEntities();db.Customers.AddObject(cust);
db.Orders.AddObject(ord);
db.SaveChanges();
更新實體類和修改對象的屬性一樣簡單：
NorthwindEntities db = new NorthwindEntities();Customer cust = (from c in db.Customerswhere c.CustomerID == "LAWN"
select c).Single();
cust.ContactName = "John Smith";cust.Fax = "(800) 123 1234";db.SaveChanges();
Single（）：返回序列的唯一元素；如果該序列并非恰好包含一個元素，則會引發(fā)異常。
刪除也很簡單：
NorthwindEntities db = new NorthwindEntities();IEnumerable<Order_Detail> ods = from o in db.Order_Details where o.OrderID == 10248select o;
// 對 LINQ 查詢而返回的結(jié)果集進行處理// 要么使用 Single() 取出單條記錄// 要么就迭代集合進行處理foreach (Order_Detail o in ods)
{db.Order_Details.DeleteObject(o);
}
db.SaveChanges();
注意：Entity Framework 不會刪除關(guān)聯(lián)的實體對象，因此調(diào)用 SaveChanges（）前必須小心地刪除所有通過外鍵約束關(guān)聯(lián)的對象。
Entity Framework 默認使用樂觀并發(fā)模型，也就是說在讀取數(shù)據(jù)后，它不檢查是否有人修改了數(shù)據(jù)庫中的數(shù)據(jù)。調(diào)用 SaveChanges（）時，所有待更新數(shù)據(jù)全部被寫到數(shù)據(jù)庫中，即使他人已經(jīng)更新了有沖突的記錄時也是如此。
樂觀并發(fā)會導(dǎo)致痛苦的數(shù)據(jù)一致性問題。
可以讓 Entity Framework 在更新前檢查數(shù)據(jù)庫是否由第三方執(zhí)行了更改，雖然這還是樂觀并發(fā)，因為實體對象不會鎖住數(shù)據(jù)庫的任何對象。但至少它可以在發(fā)生問題時給你提醒。
打開實體數(shù)據(jù)模型，選中字段，在屬性中設(shè)置“并發(fā)模式”為“Fixed”，如下圖：
為實體對象啟用并發(fā)沖突檢查后，試圖更新已經(jīng)被更新過的數(shù)據(jù)時，會得到一個 OptimisticConcurrencyException 。為了模擬并發(fā)異常，我們使用 Entity Framework 執(zhí)行更新，然后通過 ExecuteStatementInDb 方法直接執(zhí)行會造成沖突的 SQL 語句：
protected void Page_Load(object sender, EventArgs e)
{ NorthwindEntities db = new NorthwindEntities();Customer cust = db.Customers
.Where(c => c.CustomerID == "LAZYK").Select(c => c).First();
Response.Write(string.Format("Initial value {0}<br />", cust.ContactName));
// change the record outside of the entity frameworkstring sql = string.Format(@"update Customers set ContactName = 'Samuel Arthur Sanders'
where CustomerID = 'LAZYK'");
ExecuteStatementInDb(sql);
// modify the customer cust.ContactName = "John Doe"; // save the changes try {db.SaveChanges();
}
catch (OptimisticConcurrencyException) { Response.Write("Detected concurrency conflict - giving up<br />");}
finally {sql = string.Format(@"select ContactName from Customers
where CustomerID = 'LAZYK'");
string dbValue = GetStringFromDb(sql);Response.Write(string.Format("Database value: {0}<br />", dbValue));
Response.Write(string.Format("Cached value: {0}<br />", cust.ContactName));
}
}
private void ExecuteStatementInDb(string sql)
{string conStr = WebConfigurationManager.ConnectionStrings["Northwind"].ConnectionString;
SqlConnection conn = new SqlConnection(conStr); SqlCommand cmd = new SqlCommand(sql, conn); try {conn.Open();
Response.Write("Executeing Sql statement against database with ADO.NET ...<br />");cmd.ExecuteNonQuery();
Response.Write("Database updated.<br />");conn.Close();
}
catch (Exception err) {throw new ApplicationException(err.Message);
}
finally {conn.Close();
}
}
private string GetStringFromDb(string sql)
{string conStr = WebConfigurationManager.ConnectionStrings["Northwind"].ConnectionString;
SqlConnection conn = new SqlConnection(conStr); SqlCommand cmd = new SqlCommand(sql, conn); try {conn.Open();
object obj = cmd.ExecuteScalar();conn.Close();
return obj.ToString();}
catch (Exception err) {throw new ApplicationException(err.Message);
}
finally {conn.Close();
}
}
因為在 ContactName 字段上做了并發(fā)檢查，因此在 Entity Framework 更新時捕獲了這個異常，最終更新并沒有成功。
不過，我們除了要檢查數(shù)據(jù)的差異，還是想把數(shù)據(jù)寫回數(shù)據(jù)庫，這時可以調(diào)用 ObjectContext.Refresh（）來解決這一問題，可以在捕獲異常時增加這樣一條代碼：
catch (OptimisticConcurrencyException){ Response.Write("Detected concurrency conflict - giving up<br />");db.Refresh(RefreshMode.StoreWins, cust);
}
Refresh（）：
RefreshMode.StoreWins 表示用數(shù)據(jù)庫中的值更新實體對象的值
RefreshMode.ClientWins 表示用實體對象的值更新數(shù)據(jù)庫中的值
讓我們回顧一下這里所發(fā)生的一切。我們試圖向數(shù)據(jù)庫寫入在其他某處已經(jīng)被更新了的數(shù)據(jù)。Entity Framework 檢測到了并發(fā)沖突并拋出 OptimisticConcurrencyException 異常，讓我們知道發(fā)生了問題。我們用數(shù)據(jù)庫里的數(shù)據(jù)刷新修改了實體對象，它使得我們重新回到一致的狀態(tài)。
但我們的更新發(fā)生了什么？嗯，什么也沒有發(fā)生。如果我們一定要應(yīng)用自己的修改的話，就應(yīng)該使用 RefreshMode.ClientWins 枚舉值，并再次調(diào)用 SaveChanges（）：
catch (OptimisticConcurrencyException){ Response.Write("Detected concurrency conflict - giving up<br />");db.Refresh(RefreshMode.ClientWins, cust);
db.SaveChanges();
}
這一回，就好像說“我知道有并發(fā)沖突發(fā)生了，但我也堅持我的更新”那樣。為妥善的處理并發(fā)沖突時我們要指出一點：刷新實體對象時可能會有人再次修改數(shù)據(jù)，也就是說第二次調(diào)用 SaveChanges（）可能會引發(fā)另一個 OptimisticConcurrencyException 異常，為了解決這個問題，我們可以循環(huán)嘗試應(yīng)用更新：
// modify the customercust.ContactName = "John Doe";int maxAttempts = 5;bool recordsUpdated = false;
for (int i = 0; i < maxAttempts && !recordsUpdated; i++)
{ try {db.SaveChanges();
recordsUpdated = true;}
catch (Exception) {db.Refresh(RefreshMode.ClientWins, cust);
}
}
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