In today post, I will explain Code Migration in MVC application. This migration helps you to update and change the data model that you’ve created for application and you don't need to re-create or drop your database.

 

How to Apply Code First Migration

The data model in the database usually changes after developing the new application. When the data model changes, it becomes out of sync with the database. We've configured our Entity Framework in the app to drop and recreate the data model automatically. When any type of change is done in the entity classes or in the DbContext class, the existing database is deleted and the new database is created that matches the model and sees it with test data.

When the application runs in production, we do not want to lose everything each time we make changes like adding a column. The Code First Migration solved this problem by enabling the code first to update the database instead of dropping or recreating.

The procedure is following:

1. Disable the Database Initializer in the Web.Config file.

The following is the example:

<!--<contexts>

      <context type="Vag_Infotech.DAL.CollegeDbContext, Vag_Infotech">
        <databaseInitializer type="Vag_Infotech.DAL.CollegeDatabaseInitializer, Vag_Infotech"></databaseInitializer>
      </context>
</contexts>--> 

And also you need to change your Global.asax file:

// Database.SetInitializer(new CollegeDatabaseInitializer()); 

2. Change your database name in your web.config file:

<connectionStrings
  <add name="CollegeDbContext" connectionString="Data Source=(LocalDb)\v11.0;Initial Catalog=VInfotechNew;Integrated Security=SSPI" providerName="System.Data.SqlClient" />
</
connectionStrings> 

3. Open the Package Manager Console and enter this command:

enable-migration

add-migration InitialCreate 

Then, you will see the solution explorer below:

How to Edit Seed Method

Now, we add new code for updating our data model in this method. When we do not use the migration, the data model is dropped or re-created during the execution of the application each time, but now the data is retained after database changes.

Here are the steps to edit seed method:

1. Modify your configuration.cs file with this code:

namespace Vag_Infotech.Migrations
{
    using System;
    using System.Collections.Generic;
    using System.Data.Entity;
    using System.Data.Entity.Migrations;
    using System.Linq;
    using Vag_Infotech.Models;
 
    internal sealed class Configuration : DbMigrationsConfiguration<Vag_Infotech.DAL.CollegeDbContext>
    {
        public Configuration()
        {
            AutomaticMigrationsEnabled = false;
        }
 
        protected override void Seed(Vag_Infotech.DAL.CollegeDbContext context)
        {
            var New_Students = new List<Student>
            {
                new Student{FirstName="Louis",LastName="Fordsen",
                    EnrollmentDate=DateTime.Parse("2009-07-01")},
                new Student{FirstName="David ",LastName="Roger",
                    EnrollmentDate=DateTime.Parse("2009-07-01")},
                new Student{FirstName="Mike",LastName="Upson",
                    EnrollmentDate=DateTime.Parse("2009-07-05")},
            };
            New_Students.ForEach(ns => context.Students.AddOrUpdate(p => p.LastName, ns));
            context.SaveChanges();
 
            var New_Courses = new List<Course>
            {
                new Course{CourseID=201,Name="MCA",Credit=3,},
                new Course{CourseID=202,Name="M.Sc.IT",Credit=2,},
                new Course{CourseID=203,Name="M.Tech.CS",Credit=2,},
                new Course{CourseID=204,Name="M.Tech.IT",Credit=4,}
            };
            New_Courses.ForEach(ns => context.Courses.AddOrUpdate(p => p.Name, ns));
            context.SaveChanges();
 
            var New_Enrollments = new List<Enrollment>
            {
                new Enrollment{
                    StudentID= New_Students.Single(ns=>ns.FirstName=="Louis").ID,
                    CourseID=New_Courses.Single(nc=>nc.Name=="MCA").CourseID,
                    Grade=StudentGrade.A
                },
                new Enrollment{
                    StudentID= New_Students.Single(ns=>ns.FirstName=="David").ID,
                    CourseID=New_Courses.Single(nc=>nc.Name=="M.Tech.IT").CourseID,
                    Grade=StudentGrade.B
                },
                new Enrollment{
                    StudentID= New_Students.Single(ns=>ns.FirstName=="Mike").ID,
                    CourseID=New_Courses.Single(nc=>nc.Name=="M.Tech.CS").CourseID,
                    Grade=StudentGrade.D
                },
            };
 
            foreach (Enrollment et in New_Enrollments)
            {
                var NewEnrollDb = context.Enrollments.Where(
                    ns => ns.Student.ID == et.StudentID &&
                        ns.Course.CourseID == et.CourseID).SingleOrDefault();
                if (NewEnrollDb == null)
                {
                    context.Enrollments.Add(et);
                }
            }
            context.SaveChanges();
        }
    }
}

In the code above the Seed() takes the context object as an input argument and in the code the object is used to add the entities to the database. We can see how the code creates the collection for the database for each entity type and then saves it by SaveChanges() and adds it through the DbSet property. It is not necessary to call the SaveChanges() after each group of entities.

2. Build the solution

How to Execute the Migration

Here are steps:

1. Enter this command on Package Manager Console:

Update-database

2. Run the application and you’ll see the change