Entity Framework Core-数据库优先

数据库优先是EF Core会根据数据库自动创建Entity&Context，因此首先你得先创建数据库

我们通过一个Company数据库做个演示

1 SQL Server中创建数据库

在Visual Studio中打开View->SQL Sever Object Explorer，然后创建一个简单的数据库名称为Company，在数据库中创建两个表：

• Employee
• Department

Employee 表定义如下：

Department 表定义如下：

这两张表的Primary Key是Id列并且自增长

Department&Employee是一对多的关系，一个Department有多个Employee，我们在Employee表中创建DepartmentId列作为Department表的外键

创建表脚本

CREATE TABLE [dbo].[Department](
 [Id] [int] IDENTITY(1,1) NOT ,
 [Name] [varchar](50) NOT ,
CONSTRAINT [PK_Department] PRIMARY KEY CLUSTERED 
(
 [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
//Employee 表
CREATE TABLE [dbo].[Employee](
 [Id] [int] IDENTITY(1,1) NOT ,
 [DepartmentId] [int] NOT ,
 [Name] [varchar](100) NOT ,
 [Designation] [varchar](25) NOT ,
CONSTRAINT [PK_Employee] PRIMARY KEY CLUSTERED 
(
 [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
ALTER TABLE [dbo].[Employee] WITH CHECK ADD CONSTRAINT [FK_Employee_Department] FOREIGN KEY([DepartmentId])
REFERENCES [dbo].[Department] ([Id])
GO
ALTER TABLE [dbo].[Employee] CHECK CONSTRAINT [FK_Employee_Department]
GO

2 Entity Framework Core数据库连接字符串

数据库连接字符串包含了数据源以及一些必要的连接信息，EF Core需要连接字符串和数据库进行交互，指定数据库操作像新增，读取等，打开SQL Sever Object Explorer，鼠标选中数据库，右击属性，在属性窗体中找到Connection String并且复制该值

数据库连接字符串是：

Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=Company;Integrated Security=True;Connect Timeout=30;Encrypt=False;TrustServerCertificate=False;ApplicationIntent=ReadWrite;MultiSubnetFailover=False

3 .NET Core CLI Scaffold 命令

在Package Manager Console 窗体中运行CLI Scaffold命令，在Visual Studio 中打开Tools->NuGet Package Manager->Package Manager Console 菜单

运行之前请确保你在项目中已经安装了EF Core 相关工具链，运行命令如下：

dotnet ef dbcontext scaffold "Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=Company;Integrated Security=True;" Microsoft.EntityFrameworkCore.SqlServer -o Models

解释：

• 双引号内指定了数据库连接字符
• Microsoft.EntityFrameworkCore.SqlServer指定了使用SQL Server的Provider
• -o Models 指定了生成Entity & Context 类的目录名称

这个命令需要执行10-20秒，执行完之后我们可以看到生成的类

3.1 Context & Entity 类

我们发现在Models 目录下生成了context & entity类

3.2 Employee类

public partial class Employee
{
public int Id { get; set; }
public int DepartmentId { get; set; }
public string Name { get; set; } = !;
public string Designation { get; set; } = !;
public virtual Department Department { get; set; } = !;
}

3.3 Department类

public partial class Department
{
public int Id { get; set; }
public string Name { get; set; } = !;
public virtual ICollection<Employee> Employees { get; } = new List<Employee>();
}

3.4 CompanyContext类

public partial class CompanyContext : DbContext
{
public CompanyContext()
 {
 }
public CompanyContext(DbContextOptions<CompanyContext> options)
 : base(options)
 {
 }
public virtual DbSet<Department> Departments { get; set; }
public virtual DbSet<Employee> Employees { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
#warning To protect potentially sensitive information in your connection string, you should move it out of source code. You can avoid scaffolding the connection string by using the Name= syntax to read it from configuration - see https://go.microsoft.com/fwlink/?linkid=2131148. For more guidance on storing connection strings, see http://go.microsoft.com/fwlink/?LinkId=723263.
 => optionsBuilder.UseSqlServer("Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=Company;Integrated Security=True;");
protected override void OnModelCreating(ModelBuilder modelBuilder)
 {
 modelBuilder.Entity<Department>(entity =>
 {
 entity.ToTable("Department");

 entity.Property(e => e.Name)
 .HasMaxLength(50)
 .IsUnicode(false);
 });
 modelBuilder.Entity<Employee>(entity =>
 {
 entity.ToTable("Employee");

 entity.Property(e => e.Designation)
 .HasMaxLength(25)
 .IsUnicode(false);
 entity.Property(e => e.Name)
 .HasMaxLength(100)
 .IsUnicode(false);

 entity.HasOne(d => d.Department).WithMany(p => p.Employees)
 .HasForeignKey(d => d.DepartmentId)
 .OnDelete(DeleteBehavior.ClientSet)
 .HasConstraintName("FK_Employee_Department");
 });
 OnModelCreatingPartial(modelBuilder);
 }
partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
}

4 PMC Scaffold-DbContext 命令

我们能跳过dotnet cli scaffold 命令，在Package Manager Console 中使用Scaffold-DbContext 命令从数据库中创建Context & Entity 类，这个和上面的完全相同，在Package Manager Console 中运行如下命令

Scaffold-DbContext "Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=Company;Integrated Security=True;" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models

总结

这节我们主要学习 Entity Framework Core 数据库优先，以及使用脚手架命令根据数据库生成对应的Entity & Context，然而，数据库优先是不被推荐的，Microsoft建议我们使用代码优先方法 在下一节中我们将了解DbContext 类

参考资料