实体框架 – 多个 DbContext

在本章中，我们将学习当应用程序中有多个 DbContext 类时如何将更改迁移到数据库中。

• 多 DbContext 最早是在 Entity Framework 6.0 中引入的。
• 多个上下文类可能属于单个数据库或两个不同的数据库。

在我们的示例中，我们将为同一个数据库定义两个 Context 类。在以下代码中，Student 和 Teacher 有两个 DbContext 类。

public class Student {
   public int ID { get; set; }
   public string LastName { get; set; }
   public string FirstMidName { get; set; }
   public DateTime EnrollmentDate { get; set; }
}

public class MyStudentContext : DbContext {
   public MyStudentContext() : base("UniContextDB") {}
   public virtual DbSet<Student> Students { get; set; }
}

public class Teacher {
   public int ID { get; set; }
   public string LastName { get; set; }
   public string FirstMidName { get; set; }
   public DateTime HireDate { get; set; }
}

public class MyTeacherContext : DbContext {
   public MyTeacherContext() : base("UniContextDB") {}
   public virtual DbSet<Teacher> Teachers { get; set; }
}

正如您在上面的代码中看到的，有两个模型，分别称为“Student”和“Teacher”。每一个都与一个特定的对应上下文类相关联，即，Student 与 MyStudentContext 相关联，Teacher 与 MyTeacherContext 相关联。

当同一个项目中有多个 Context 类时，这是迁移数据库更改的基本规则。

• enable-migrations -ContextTypeName <DbContext-Name-with-Namespaces> MigrationsDirectory:<Migrations-Directory-Name>

• Add-Migration -configuration <DbContext-Migrations-Configuration-Class-withNamespaces> <Migrations-Name>

• Update-Database -configuration <DbContext-Migrations-Configuration-Class-withNamespaces> -Verbose

让我们通过在包管理器控制台中执行以下命令来为 MyStudentContext 启用迁移。

PM→ enable-migrations -ContextTypeName:EFCodeFirstDemo.MyStudentContext

执行后，我们将在迁移历史记录中添加模型，为此，我们必须在同一控制台中触发 add-migration 命令。

PM→ add-migration -configuration EFCodeFirstDemo.Migrations.Configuration Initial

现在让我们将一些数据添加到数据库中的 Students 和 Teachers 表中。

static void Main(string[] args) {

   using (var context = new MyStudentContext()) {
	
      //// Create and save a new Students
      Console.WriteLine("Adding new students");

      var student = new Student {
         FirstMidName = "Alain", 
         LastName = "Bomer", 
         EnrollmentDate = DateTime.Parse(DateTime.Today.ToString())
         //Age = 24
      };

      context.Students.Add(student);

      var student1 = new Student {
         FirstMidName = "Mark",
         LastName = "Upston", 
         EnrollmentDate = DateTime.Parse(DateTime.Today.ToString())
         //Age = 30
      };

      context.Students.Add(student1);
      context.SaveChanges();
		
      // Display all Students from the database
      var students = (from s in context.Students orderby s.FirstMidName
         select s).ToList<Student>();
		
      Console.WriteLine("Retrieve all Students from the database:");

      foreach (var stdnt in students) {
         string name = stdnt.FirstMidName + " " + stdnt.LastName;
         Console.WriteLine("ID: {0}, Name: {1}", stdnt.ID, name);
      }

      Console.WriteLine("Press any key to exit...");
      Console.ReadKey();
   }

   using (var context = new MyTeacherContext()) {

      //// Create and save a new Teachers
      Console.WriteLine("Adding new teachers");

      var student = new Teacher {
         FirstMidName = "Alain", 
         LastName = "Bomer", 
         HireDate = DateTime.Parse(DateTime.Today.ToString())
         //Age = 24
      };

      context.Teachers.Add(student);

      var student1 = new Teacher {
         FirstMidName = "Mark", 
         LastName = "Upston", 
         HireDate = DateTime.Parse(DateTime.Today.ToString())
         //Age = 30
      };

      context.Teachers.Add(student1);
      context.SaveChanges();
  
      // Display all Teachers from the database
      var teachers = (from t in context.Teachers orderby t.FirstMidName
         select t).ToList<Teacher>();
		
      Console.WriteLine("Retrieve all teachers from the database:");

      foreach (var teacher in teachers) {
         string name = teacher.FirstMidName + " " + teacher.LastName;
         Console.WriteLine("ID: {0}, Name: {1}", teacher.ID, name);
      }

      Console.WriteLine("Press any key to exit...");
      Console.ReadKey();
   }
}

执行上述代码时，您将看到为两个不同的模型创建了两个不同的表，如下图所示。

我们建议您逐步执行上述示例，以便更好地理解。