问题
你想不用Include()方法,立即加载一下相关的集合,并想通过EF的CodeFirst方式实现.
解决方案
假设你有一个如Figure 13-14所示的模型:
Figure 13-14. A model for a Customer, their CreditCards, and Transactions
首先本例通过EF的CodeFrist方式实现,代码Listing 13-23,我们创建Customer,CreditCard和Transaction实体类.
Listing 13-23. The Reservation Entity Object
public class Customer
{
public Customer()
{
CreditCards = new HashSet<CreditCard>();
}
public int CustomerId { get; set; }
public string Name { get; set; }
public string City { get; set; }
public virtual ICollection<CreditCard> CreditCards { get; set; }
}
public class CreditCard
{
public CreditCard()
{
Transactions = new HashSet<Transaction>();
}
public string CardNumber { get; set; }
public string Type { get; set; }
public System.DateTime ExpirationDate { get; set; }
public int CustomerId { get; set; }
public virtual Customer Customer { get; set; }
public virtual ICollection<Transaction> Transactions { get; set; }
}
public class Transaction
{
public int TransactionId { get; set; }
public string CardNumber { get; set; }
public decimal Amount { get; set; }
public virtual CreditCard CreditCard { get; set; }
}
接下来,代码Listing 13-24,我们创建DbObject对象,通过它访问EF的功能.
Listing 13-24. DbContext Object
public class Recipe9Context : DbContext
{
public Recipe9Context()
: base("Recipe9ConnectionString")
{
// Disable Entity Framework Model Compatibility
Database.SetInitializer<Recipe10Context>(null);
}
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
// explicilty specify primary key for CreditCard
modelBuilder.Entity<CreditCard>().HasKey(x => x.CardNumber);
modelBuilder.Entity<Customer>().ToTable("Chapter13.Customer");
modelBuilder.Entity<CreditCard>().ToTable("Chapter13.CreditCard");
modelBuilder.Entity<Transaction>().ToTable("Chapter13.Transaction");
}
public DbSet<CreditCard> CreditCards { get; set; }
public DbSet<Customer> Customers { get; set; }
public DbSet<Transaction> Transactions { get; set; }
}
接下来在项目中添加App.config,并把代码Listing13-25 添加到ConnectionStrings节下.
Listing 13-25. Connection String
<connectionStrings>
<add name="Recipe9ConnectionString"
connectionString="Data Source=.;
Initial Catalog=EFRecipes;
Integrated Security=True;
MultipleActiveResultSets=True"
providerName="System.Data.SqlClient" />
</connectionStrings>
想通过指定的City把Customers和相关的CreditCards以及Transactions都加载,但不用Include()方法.式加载实体和让EF维护关联,如代码Listing13-26所示
Listing 13-26. Loading Related Entities without Using Include()
using (var context = new Recipe9Context())
{
var cust1 = new Customer { Name = "Robin Rosen", City = "Raytown" };
var card1 = new CreditCard
{
CardNumber = "41949494338899",
ExpirationDate = DateTime.Parse("12/2010"),
Type = "Visa"
};
var trans1 = new Transaction { Amount = 29.95M };
card1.Transactions.Add(trans1);
cust1.CreditCards.Add(card1);
var cust2 = new Customer { Name = "Bill Meyers", City = "Raytown" };
var card2 = new CreditCard
{
CardNumber = "41238389484448",
ExpirationDate = DateTime.Parse("12/2013"),
Type = "Visa"
};
var trans2 = new Transaction { Amount = 83.39M };
card2.Transactions.Add(trans2);
cust2.CreditCards.Add(card2);
context.Customers.Add(cust1);
context.Customers.Add(cust2);
context.SaveChanges();
}
using (var context = new Recipe9Context())
{
var customers = context.Customers.Where(c => c.City == "Raytown");
var creditCards = customers.SelectMany(c => c.CreditCards);
var transactions = creditCards.SelectMany(cr => cr.Transactions);
// 执行查询,EF会维护关联
customers.ToList();
creditCards.ToList();
transactions.ToList();
foreach (var customer in customers)
{
Console.WriteLine("Customer: {0} in {1}", customer.Name, customer.City);
foreach (var creditCard in customer.CreditCards)
{
Console.WriteLine(" Card: {0} expires on {1}",
creditCard.CardNumber, creditCard.ExpirationDate.ToShortDateString());
foreach (var trans in creditCard.Transactions)
{
Console.WriteLine(" Transaction: {0}", trans.Amount.ToString("C"));
}
}
}
}
输出结果如下:
Customer: Robin Rosen in Raytown
Card: 41949494338899 expires on 12/1/2010
Transaction: $29.95
Customer: Bill Meyers in Raytown
Card: 41238389484448 expires on 12/1/2013
Transaction: $83.39
它是如何工作
Inclue()方法是一个强大的和通常有效的急切加载相关的实体的工具.然而Include()确实存在一些性能缺陷.虽然使用Include()能一次性从数据库返回结果,但如代码Listing 13-26用三个查询来代码,因为用一个查询可能会使查询很复杂,在某些情况下,一个查询不一定能像三个简单查询那样,执行得那么好.另外,单个的复杂的查询返回的结果集包含更多的列,增加通过线缆传送数据的负担.规则是:在你的查询中越多的Include,就得付出越高的性能代价.
另一方面,不用Include()方法或迭代超过一定数量的Customers,我们创建太多的小查询,结果也会导致大量的性能损失.第5单讨论了获取明细的平衡做法.