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Entity?Framework代碼優(yōu)先(Code?First)模式

 更新時間：2022年06月13日 14:35:11   作者：springsnow  
這篇文章介紹了Entity?Framework代碼優(yōu)先(Code?First)模式，文中通過示例代碼介紹的非常詳細。對大家的學習或工作具有一定的參考借鑒價值，需要的朋友可以參考下

一、Code First 代碼優(yōu)先

DbContext可以用于數(shù)據(jù)庫優(yōu)先，代碼優(yōu)先和模型優(yōu)先的開發(fā)。

DbContext主要包含一組非常易于使用的API。該API由ObjectContext公開。這些API還允許我們使用ObjectContext不允許的Code First方法。

DbContext只是ObjectContext包裝器，可以說它是ObjectContext的輕量級替代方案。

1、從ObjectContext轉(zhuǎn)化到DbContext

DbContext ctx= new DbContext(ctxObj , true);

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

二、創(chuàng)建或生成Model代碼

1、從數(shù)據(jù)庫生成Model代碼

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

一般使用Visual Studio EF 6工具附帶的不太高級的“"Code First from Database" ”功能。

實體框架提供了一種對現(xiàn)有數(shù)據(jù)庫使用代碼優(yōu)先方法的簡便方法。它將為現(xiàn)有數(shù)據(jù)庫中的所有表和視圖創(chuàng)建實體類，并使用數(shù)據(jù)注釋屬性和Fluent API對其進行配置。

要將代碼優(yōu)先用于現(xiàn)有數(shù)據(jù)庫，請在Visual Studio中右鍵單擊您的項目->添加->新建項。

在“添加新項”對話框中選擇“ADO.NET實體數(shù)據(jù)模型”，并指定模型名稱（這將是上下文類名稱），然后單擊“添加”。

這將打開“實體數(shù)據(jù)模型”向?qū)В缦滤?。從?shù)據(jù)庫選項中選擇代碼優(yōu)先，然后單擊下一步。

現(xiàn)在，為現(xiàn)有數(shù)據(jù)庫選擇數(shù)據(jù)連接。如果下拉列表不包括與現(xiàn)有數(shù)據(jù)庫的連接，請為您的數(shù)據(jù)庫創(chuàng)建一個新連接。單擊下一步繼續(xù)。

現(xiàn)在，選擇要為其生成類的表和視圖，然后單擊“完成”。

這將為您的數(shù)據(jù)庫表和視圖生成所有實體類，如下所示。

例如，它將創(chuàng)建以下上下文類，該上下文類使用Fluent API根據(jù)數(shù)據(jù)庫配置實體類。

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有用的設計時實用程序：https://marketplace.visualstudio.com/items?itemName=ErikEJ.EntityFramework6PowerToolsCommunityEdition

2、手工創(chuàng)建Model代碼

方式1、使用標注

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

namespace CodeFirst.Model
{
    /// 

    /// 目的景點類
    /// 
    [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; } //景點帶有多個住宿點
    }

    /// 

    /// 住宿類
    /// 
    [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; }
    }

    /// 

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

    /// 

    /// 旅館類,繼承自住宿類
    /// 
    public class Hostel : Lodging
    {
        [Column("MAXROOM", TypeName = "INT")]
        public int? MaxRoom { get; set; }
    }
}

方式2：使用模板Builder“配置”屬性和關(guān)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、添加單個實體，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、刪除，Remove

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

五、查詢

1、Load()：

把數(shù)據(jù)加載到內(nèi)存，使用實體的Local屬性訪問。（LINQ寫法，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()：

一次性從數(shù)據(jù)庫中查出數(shù)據(jù)

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

3、Find():

根據(jù)鍵值先從內(nèi)存中查詢，內(nèi)存中沒有才查詢數(shù)據(jù)庫。

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

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

可根據(jù)條件直接從數(shù)據(jù)庫查。

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

六、直接執(zhí)行SQL語句

1、在實體上運行SQL命令，

SQL查詢：SqlQuery

DbSqlQuery c = ctx.Lodging.SqlQuery("select * from.."); //EF跟蹤返回的對象。

2、在Database屬性上運行SQL命令

1、SQL查詢：Database.SqlQuery

IEnumerable a = ctx.Database.SqlQuery("Select * from.."); //可直接轉(zhuǎn)為定義的實體類型，任何類型，EF不跟蹤

2、執(zhí)行SQL命令：Database.ExecuteSqlCommand

ctx.Database.ExecuteSqlCommand("delete from pams.DESTINATIONS where Name='Bali'");//直接執(zhí)行sql

到此這篇關(guān)于Entity Framework代碼優(yōu)先(Code First)的文章就介紹到這了。希望對大家的學習有所幫助，也希望大家多多支持腳本之家。

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