Entity Framework（03）：Code First基础

一、Code First 代码优先

DbContext可以用于数据库优先，代码优先和模型优先的开发。

DbContext主要包含一组非常易于使用的API。该API由ObjectContext公开。这些API还允许我们使用ObjectContext不允许的Code First方法。

DbContext只是ObjectContext包装器，可以说它是ObjectContext的轻量级替代方案。

1、从ObjectContext转化到DbContext

DbContext ctx= new DbContext(ctxObj , true);

EF6支持Oracle ODT 12C Release 3 (net4.5)以上

二、创建或生成Model代码

1、从数据库生成Model代码

可以使用高级的反向工程工具POCO生成器模板（收费）。https://marketplace.visualstudio.com/items?itemName=SimonHughes.EntityFrameworkReversePOCOGenerator

一般使用Visual Studio EF 6工具附带的不太高级的“"Code First from Database" ”功能。

实体框架提供了一种对现有数据库使用代码优先方法的简便方法。它将为现有数据库中的所有表和视图创建实体类，并使用数据注释属性和Fluent API对其进行配置。

要将代码优先用于现有数据库，请在Visual Studio中右键单击您的项目->添加->新建项。


code first for an existing database

在“添加新项”对话框中选择“ADO.NET实体数据模型”，并指定模型名称（这将是上下文类名称），然后单击“添加”。

code first for an existing database

这将打开“实体数据模型”向导，如下所示。从数据库选项中选择代码优先，然后单击下一步。

code first for an existing database

现在，为现有数据库选择数据连接。如果下拉列表不包括与现有数据库的连接，请为您的数据库创建一个新连接。单击下一步继续。

code first for an existing database

现在，选择要为其生成类的表和视图，然后单击“完成”。

code first for an existing database

这将为您的数据库表和视图生成所有实体类，如下所示。

code first for an existing database

例如，它将创建以下上下文类，该上下文类使用Fluent API根据数据库配置实体类。

namespace EFDemo
{
    using System;
    using System.Data.Entity;
    using System.ComponentModel.DataAnnotations.Schema;
    using System.Linq;

    public partial class SchoolContext : DbContext
    {
        public SchoolContext()
            : base("name=SchoolContext2")
        {
        }

        public virtual DbSet<Course> Courses { get; set; }
        public virtual DbSet<Standard> Standards { get; set; }
        public virtual DbSet<Student> Students { get; set; }
        public virtual DbSet<StudentAddress> StudentAddresses { get; set; }
        public virtual DbSet<Teacher> Teachers { get; set; }
        public virtual DbSet<View_StudentCourse> View_StudentCourse { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Course>()
                .Property(e => e.CourseName)
                .IsUnicode(false);

            modelBuilder.Entity<Course>()
                .HasMany(e => e.Students)
                .WithMany(e => e.Courses)
                .Map(m => m.ToTable("StudentCourse").MapLeftKey("CourseId").MapRightKey("StudentId"));

            modelBuilder.Entity<Standard>()
                .Property(e => e.StandardName)
                .IsUnicode(false);

            modelBuilder.Entity<Standard>()
                .Property(e => e.Description)
                .IsUnicode(false);

            modelBuilder.Entity<Standard>()
                .HasMany(e => e.Students)
                .WithOptional(e => e.Standard)
                .WillCascadeOnDelete();

            modelBuilder.Entity<Standard>()
                .HasMany(e => e.Teachers)
                .WithOptional(e => e.Standard)
                .WillCascadeOnDelete();

            modelBuilder.Entity<Student>()
                .Property(e => e.StudentName)
                .IsUnicode(false);

            modelBuilder.Entity<Student>()
                .Property(e => e.RowVersion)
                .IsFixedLength();

            modelBuilder.Entity<Student>()
                .HasOptional(e => e.StudentAddress)
                .WithRequired(e => e.Student)
                .WillCascadeOnDelete();

            modelBuilder.Entity<StudentAddress>()
                .Property(e => e.Address1)
                .IsUnicode(false);

            modelBuilder.Entity<StudentAddress>()
                .Property(e => e.Address2)
                .IsUnicode(false);

            modelBuilder.Entity<StudentAddress>()
                .Property(e => e.City)
                .IsUnicode(false);

            modelBuilder.Entity<StudentAddress>()
                .Property(e => e.State)
                .IsUnicode(false);

            modelBuilder.Entity<Teacher>()
                .Property(e => e.TeacherName)
                .IsUnicode(false);

            modelBuilder.Entity<Teacher>()
                .HasMany(e => e.Courses)
                .WithOptional(e => e.Teacher)
                .WillCascadeOnDelete();

            modelBuilder.Entity<View_StudentCourse>()
                .Property(e => e.StudentName)
                .IsUnicode(false);

            modelBuilder.Entity<View_StudentCourse>()
                .Property(e => e.CourseName)
                .IsUnicode(false);
        }
    }
}

EF 6有用的设计时实用程序：https://marketplace.visualstudio.com/items?itemName=ErikEJ.EntityFramework6PowerToolsCommunityEdition

image

2、手工创建Model代码

方式1、使用标注

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;

namespace CodeFirst.Model
{
    /// 
    /// 目的景点类
    /// 
    [Table("DESTINATIONS", Schema = "PAMS")]
    public class Destination
    {
        [Column("DESTINATIONID", TypeName = "INT")]
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public int DestinationId { get; set; }

        [Column("NAME")]
        public string Name { get; set; }

        [Column("COUNTRY")]
        public string Country { get; set; }

        [Column("DESCRIPTION")]
        public string Description { get; set; }

        [Column("PHOTO")]
        public byte[] Photo { get; set; }

        public virtual List Lodgings { get; set; } //景点带有多个住宿点
    }

    /// 
    /// 住宿类
    /// 
    [Table("LODGINGS", Schema = "PAMS")]
    public class Lodging
    {
        [Column("LODGINGID", TypeName = "INT")]
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public int LodgingId { get; set; }

        [Column("NAME")]
        public string Name { get; set; }

        [Column("OWNER")]
        public string Owner { get; set; }

        [Column("TARDESTINATIONID", TypeName = "INT")]
        public int? DestinationID { get; set; }

        [ForeignKey("DestinationID")]
        public Destination Destination { get; set; }
    }

    /// 
    /// 度假村类,继承自住宿类
    /// 
    public class Resort : Lodging
    {
        [Column("ENTERTAINMENT")]
        public string Entertainment { get; set; }
    }

    /// 
    /// 旅馆类,继承自住宿类
    /// 
    public class Hostel : Lodging
    {
        [Column("MAXROOM", TypeName = "INT")]
        public int? MaxRoom { get; set; }
    }
}

方式2：使用模板Builder“配置”属性和关系

using CodeFirst.Model;
using System.Data.Entity;
using System.Data.Entity.ModelConfiguration;

namespace CodeFirst.DataAccess
{
    public class BreakAwayContext : DbContext
    {
        public DbSet Destinations { get; set; }
        public DbSet Lodgings { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Configurations.Add(new DestinationMap());
            modelBuilder.Configurations.Add(new LodgingMap());
        }
    }

    public class DestinationMap : EntityTypeConfiguration
    {
        public DestinationMap()
        {
            this.HasKey(t => t.DestinationId);
            Property(d => d.Name).IsRequired();
            Property(d => d.Description).HasMaxLength(500);
        }
    }

    public class LodgingMap : EntityTypeConfiguration
    {
        public LodgingMap()
        {
            this.HasKey(t => t.LodgingId);
            Property(d => d.Name).IsRequired();
            Property(d => d.Owner).HasMaxLength(500);
            this.Map(d => d.Requires("TYPE").HasValue("Standard"));
            this.Map(d => d.Requires("TYPE").HasValue("Resort"));
            this.Map(d => d.Requires("TYPE").HasValue("Hostel"));
        }
    }
}

三、配置文件

<connectionStrings>
    <add name="BreakAwayContext" connectionString="Data Source=(localdb)MSSQLLocalDB;Initial Catalog=BreakAway;Integrated Security=True;Connect Timeout=30;Encrypt=False;TrustServerCertificate=False;ApplicationIntent=ReadWrite;MultiSubnetFailover=False" providerName="System.Data.SqlClient"/>
</connectionStrings>

四、操作

1、添加单个实体，Add

var destination = new CodeFirst.Model.Destination
{
    Country = "Indonesia",
    Description = "EcoTourism at its best in exquisite Bali",
    Name = "Bali"
};

using (var context = new CodeFirst.DataAccess.BreakAwayContext())
{
    if (context.Destinations.Count((t => t.Name == "Bali") < 1)
    {
        context.Destinations.Add(destination);
        context.SaveChanges();
    }
}

2、修改

using (var context = new CodeFirst.DataAccess.BreakAwayContext())
{
    var canyon = (from d in context.Destinations where d.Name == "Bali" select d).Single();
    canyon.Description = "227 mile long canyon.";
    //context.Entry(canyon )=EntityState.Modified;
    context.SaveChanges();
}

3、删除，Remove

var toDelete = new CodeFirst.Model.Destination { Name = "Bali" };
context.Destinations.Attach(toDelete);  //attach
context.Destinations.Remove(toDelete);
context.SaveChanges();

五、查询

1、Load()：

把数据加载到内存，使用实体的Local属性访问。（LINQ写法，同foreach）

using (var context = new CodeFirst.DataAccess.BreakAwayContext())
{
    var query = from d in context.Destinations where d.Country == "Australia" select d;
    query.Load();//    foreach 也可以
    var count = context.Destinations.Local.Count;
}

2、ToList()：

一次性从数据库中查出数据

var Invoices=ctx.Invoie.ToList();
foreach(var result in Onvoices)
{.}

3、Find():

根据键值先从内存中查询，内存中没有才查询数据库。

var destination = context.Destinations.Find(4);

4、Single()、SingleOrDefault()、First()等:

可根据条件直接从数据库查。

var destination = context.Destinations.SingleOrDefault(d => d.Name == "Bali");

六、直接执行SQL语句

1、在实体上运行SQL命令，

SQL查询：SqlQuery 

DbSqlQuery c = ctx.Lodging.SqlQuery("select * from.."); //EF跟踪返回的对象。

2、在Database属性上运行SQL命令

1、SQL查询：Database.SqlQuery 

IEnumerable a = ctx.Database.SqlQuery("Select * from.."); //可直接转为定义的实体类型，任何类型，EF不跟踪

2、执行SQL命令：Database.ExecuteSqlCommand

ctx.Database.ExecuteSqlCommand("delete from pams.DESTINATIONS where Name='Bali'");//直接执行sql
【推广】 免费学中医，健康全家人
原文地址：https://www.cnblogs.com/springsnow/p/13230074.html