使用EF 4.1的DbContext(上)

简述：EF4.1包括Code First和DbContext API。DbContext API为EF提供更多的工作方式：Code First，Database First和Model First。
使用DbContext构造函数
1. Code First约定连接
namespace Magic.Unicorn
{
    public class UnicornsContext : DbContext
    {
        public UnicornsContext()
        // C# will call base class parameterless constructor by default
        {
        }
    }
}
用Magic.Unicorn.UnicornsContext作为数据库名，在本机上生成该数据库的连接字符串（SQL Express）。
 
2. Code First指定数据库名称的约定连接
public class UnicornsContext : DbContext
{
    public UnicornsContext()
        : base("UnicornsDatabase")
    {
    }
}
用UnicornsDatabase作为数据库名，在本机上生成该数据库的连接字符串（SQL Express）。
 
3. Code First用配置文件中的连接字符串
<connectionStrings>
    <add name="UnicornsCEDatabase"
         providerName="System.Data.SqlServerCe.4.0"
         connectionString="Data Source=Unicorns.sdf"/>
  </connectionStrings>
public class UnicornsContext : DbContext
{
    public UnicornsContext()
        : base("name=UnicornsCEDatabase")
    {
    }
}
 
4. Database/Model First用配置文件中的连接字符
<add name="Northwind_Entities"
   connectionString="
      metadata=res://*/Northwind.csdl|
               res://*/Northwind.ssdl|
               res://*/Northwind.msl;
      provider=System.Data.SqlClient;
      provider connection string=
         &quot;Data Source=.\sqlexpress;
               Initial Catalog=Northwind;
               Integrated Security=True;
               MultipleActiveResultSets=True&quot;"
providerName="System.Data.EntityClient"/>
public class NorthwindContext : DbContext
{
    public NorthwindContext()
        : base("name=Northwind_Entities")
    {
    }
}
5. Defining sets on a derived context
IDbSet Represents the collection of all entities in the context, or that can be queried from the database, of a given type. DbSet is a concrete implementation of IDbSet(Of TEntity).
1) DbSet属性：指定集合为Entity类型
2) IDbSet属性
3) 只读set属性
public IDbSet<Unicorn> Unicorns{get{return Set<Unicorn>();}
6. 利用主键查找实体
   DbSet的Find方法，如果用主键在上下文中查找不到实体，就会到数据库中查询。
   1）通过主键查找实体
      var unicorn=context.Unicorns.Find(3);//查询数据库
      var unicornAgain=context.Unicorns.Find(3);//从当前上下文中返回相同的实例（没有查询数据库）
   2) 通过主键和外键查找实体
      var lady = context.LadiesInWaiting.Find(3, "The EF Castle");
7. 实体状态和保存
   EntityState：Added、Unchanged、Modified、Deleted和Detached
   1) 插入一个新的实体到上下文
      当调用SaveChanges()才能插入到数据库中。
    var unicorn = new Unicorn { Name = "Franky", PrincessId = 1};
    context.Unicorns.Add(unicorn);//添加到上下文中
    context.SaveChanges();//插入到数据库中
     另一种插入方法：
     var unicorn = new Unicorn { Name = "Franky", PrincessId = 1};
     context.Entry(unicorn).State = EntityState.Added;
     context.SaveChanges();
     在当前实体的关联实体中添加新的对象：
     // Add a new princess by setting a reference from a tracked unicorn
      var unicorn = context.Unicorns.Find(1);
      unicorn.Princess = new Princess { Name = "Belle" };
      // Add a new unicorn by adding to the collection of a tracked princess
      var princess = context.Princesses.Find(2);
      princess.Unicorns.Add(new Unicorn { Name = "Franky" });
      context.SaveChanges();
8. DbContext中的T-SQL查询
     1) SQL语句查询实体
      var unicorns = context.Unicorns.SqlQuery(
                   "select * from Unicorns").ToList();
     2) SQL语句查询非实体类型
      var unicornNames = context.Database.SqlQuery<string>(
                       "select Name from Unicorns").ToList();
     3) 执行SQL命令
       context.Database.ExecuteSqlCommand(
        "update Unicorns set Name = 'Franky' where Name = 'Beepy'");