Code First Migrations With Entity Framework

using System;  
namespace EF.Core  
{  
   public class Student  
   {  
      public Int64 Id { get; set; }  
      public string Name { get; set; }  
      public int Age { get; set; }  
   }  
}  

using System.ComponentModel.DataAnnotations.Schema;  
using System.Data.Entity.ModelConfiguration;  
using EF.Core;  
  
namespace EF.Data.Mapping  
{  
  public class StudentMap : EntityTypeConfiguration<Student>  
    {  
      public StudentMap()  
      {  
          //key  
          HasKey(t => t.Id);  
          //property  
          Property(t => t.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);  
          Property(t => t.Name);  
          Property(t => t.Age);          
          //table  
          ToTable("Students");  
      }  
    }  
}  

<configuration>
  <connectionStrings>
    <add name="DbConnectionString" connectionString="Data Source=sandeepss-PC;Initial Catalog=EFCodeFirst;User ID=sa; Password=****" providerName="System.Data.SqlClient" />
 </connectionStrings>
</configuration>

using System.Data.Entity;
using EF.Data.Mapping;
namespace EF.Data
{
    public class EFDbContext : DbContext
    {
        public EFDbContext()
            : base("name=DbConnectionString")
        {
        }

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

    }
}

using System;
using EF.Core;
using EF.Data;
namespace EF.Console
{
    class Program
    {
        static void Main(string[] args)
        {
            System.Console.Write("Enter your name: ");
            string name = System.Console.ReadLine();
            System.Console.Write("Enter your age: ");
            int age = 0;
            Int32.TryParse(System.Console.ReadLine(), out age);
            using (EFDbContext context = new EFDbContext())
            {
                Student student = new Student { Name = name, Age = age };
                context.Entry(student).State = System.Data.EntityState.Added;
                context.SaveChanges();
            }
            System.Console.ReadLine();
        }
    }
}

Now we run the preceding code and get the result that the database was created in SQL Server and inserted a row into the Student table.

Figure 2. Database Created

Now we update the data model by adding a new field IsCurrent in it so the update of the Student class will be such as.

using System;
namespace EF.Core
{
    public class Student
    {
        public Int64 Id { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
        public bool IsCurrent { get; set; }
    }
}

As per the preceding explanation we also need to update its configuration class StudentMap such as.

using System.ComponentModel.DataAnnotations.Schema;  
using System.Data.Entity.ModelConfiguration;  
using EF.Core;  
namespace EF.Data.Mapping  
{  
    public class StudentMap : EntityTypeConfiguration<Student>  
    {  
        public StudentMap()  
        {  
            //key  
            HasKey(t => t.Id);  
            //property  
            Property(t => t.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);  
            Property(t => t.Name);  
            Property(t => t.Age);  
            Property(t => t.IsCurrent);  
            //table  
            ToTable("Students");  
        }  
    }  
}  

Thereafter we update the Main method of Program.cs so that we can insert a value for the student as the current student or not.

using System;  
using EF.Core;  
using EF.Data;  
namespace EF.Console  
{  
    class Program  
    {  
        static void Main(string[] args)  
        {  
            System.Console.Write("Enter your name : ");  
            string name = System.Console.ReadLine();  
            System.Console.Write("Enter your age : ");  
            int age = 0;  
            Int32.TryParse(System.Console.ReadLine(), out age);  
            System.Console.Write("You are current student");  
            bool isCurrent = System.Console.ReadLine() == "Yes" ? true : false;  
            using (EFDbContext context = new EFDbContext())  
            {  
                Student student = new Student { Name = name, Age = age, IsCurrent = isCurrent };  
                context.Entry(student).State = System.Data.EntityState.Added;  
                context.SaveChanges();  
            }  
            System.Console.ReadLine();  
        }  
    }  
}  

Now we run the application and get this error.

Figure 3. Error in adding a record to the database

The preceding error shows that the data model has been changed since the database was created and it's right. We have two solutions to resolve this error, either delete the database or use migrations. The first one is not useful since in that case, we lose all the data from the database so we will see the second solution in the next section.

Code first migrations

The Code First Migration is used to update the database. Here we will look at how to use it in our application. Let's see it step-by-step.

From the Tools menu, click Library Package Manager and then Package Manager Console then choose the default project EF.Data in it. That means always choosing the project with your context class for migrations.

At the PM> prompt enter the following command:

PM> enable-migrations

When running the preceding command you will get a console window such as:

Figure 4. Enable code first migration

This command adds a new folder, Migrations, in the project EF.Data and this folder contain a configuration file with default settings.

Now we add to the configuration settings in the Configuration class constructor, one to allow migration and another for no data loss when migrating. The excerpt of the Configuration class for these properties is.

AutomaticMigrationsEnabled = true;
AutomaticMigrationDataLossAllowed = false;

We set the AutomaticMigrationEnabled property to true; that means we are using automatic code first migration and another property AutomaticMigrationDataLossAllowed is set to false. That means that during the migration no existing data is lost from that migration of the table of the database. The entire Configuration class is as follows.

namespace EF.Data.Migrations
{
    using System;
    using System.Data.Entity;
    using System.Data.Entity.Migrations;
    using System.Linq;
    internal sealed class Configuration : DbMigrationsConfiguration<EF.Data.EFDbContext>
    {
        public Configuration()
        {
            AutomaticMigrationsEnabled = true;
            AutomaticMigrationDataLossAllowed = false;
        }
        protected override void Seed(EF.Data.EFDbContext context)
        {
            // This method will be called after migrating to the latest version.

            // You can use the DbSet<T>.AddOrUpdate() helper extension method
            // to avoid creating duplicate seed data. E.g.
            // context.People.AddOrUpdate(
            //   p => p.FullName,
            //   new Person { FullName = "Andrew Peters" },
            //   new Person { FullName = "Brice Lambson" },
            //   new Person { FullName = "Rowan Miller" }
            // );
        }
    }
}

The seed method is used to insert default values into the database table.

Thereafter we will update the database using the Package Manager Console. To update the database at the PM> prompt enters the following command:

PM> Update-Database -Verbose

The "-Verbose" flag specifies to show the SQL statements being applied to the target database in the console. You get results as in the following figure in the Package Manager Console.

Figure 5. Update existing database

Now we check the database and add a new record using our console application. We find that in the database there is no data loss and the application is running smoothly without throwing any exception. You can write the following query in the database and get the results as in Figure 5.

SELECT [Id], [Name], [Age], [IsCurrent]
FROM [EFCodeFirst].[dbo].[Students]

Figure 6. Retrieve data from the student table.

Conclusion

This article introduced Code First Migrations using Entity Framework. I hope that these two articles will provide a clearer understanding of how to use the Entity Framework in our application. If you have any concerns, post as a comment.