使用EF 4.1的DbContext
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)。
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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)。
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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")
??? {
??? }
}
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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=
???????? "Data Source=.\sqlexpress;
?????????????? Initial Catalog=Northwind;
?????????????? Integrated Security=True;
?????????????? MultipleActiveResultSets=True""
providerName="System.Data.EntityClient"/>
public class NorthwindContext : DbContext
{
??? public NorthwindContext()
??????? : base("name=Northwind_Entities")
??? {
??? }
}
5. Defining sets on a derived context
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'");