Move Configurations to Separate Class in Code-First:

By now, we have configured all the domain classes in OnModelCreating method in the previous sections. When you have a large number of domain classes, then configuring every class in OnModelCreating can become unmanageable. Code-First enables you to move all the configurations related to one domain class to a separate class.

In the below example, we configured Student entity.

public class SchoolDBContext: DbContext
{
    public SchoolDBContext(): base()
    {
    }

    public DbSet<Student> Students { get; set; }
    public DbSet<Standard> Standards { get; set; }
    public DbSet<StudentAddress> StudentAddress { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
            modelBuilder.Entity<Student>().ToTable("StudentInfo");

            modelBuilder.Entity<Student>().HasKey<int>(s => s.StudentKey);

            modelBuilder.Entity<Student>()
                    .Property(p => p.DateOfBirth)
                    .HasColumnName("DoB")
                    .HasColumnOrder(3)
                    .HasColumnType("datetime2");

            modelBuilder.Entity<Student>()
                    .Property(p => p.StudentName)
                    .HasMaxLength(50);

                modelBuilder.Entity<Student>()
                    .Property(p => p.StudentName)
                    .IsConcurrencyToken();

            modelBuilder.Entity<Student>()
                .HasMany<Course>(s => s.Courses)
                .WithMany(c => c.Students)
                .Map(cs =>
                        {
                            cs.MapLeftKey("StudentId");
                            cs.MapRightKey("CourseId");
                            cs.ToTable("StudentCourse");
                        });
    }
}

Now, you can move all the configurations related to Student entity to a separate class which derives from EntityTypeConfiguration<TEntity>. Consider the following StudentEntityConfigurations class.

public class StudentEntityConfiguration: EntityTypeConfiguration<Student>
{
    public StudentEntityConfiguration()
    {

            this.ToTable("StudentInfo");

            this.HasKey<int>(s => s.StudentKey);

            this.Property(p => p.DateOfBirth)
                    .HasColumnName("DoB")
                    .HasColumnOrder(3)
                    .HasColumnType("datetime2");

            this.Property(p => p.StudentName)
                    .HasMaxLength(50);

            this.Property(p => p.StudentName)
                    .IsConcurrencyToken();

            this.HasMany<Course>(s => s.Courses)
                .WithMany(c => c.Students)
                .Map(cs =>
                        {
                            cs.MapLeftKey("StudentId");
                            cs.MapRightKey("CourseId");
                            cs.ToTable("StudentCourse");
                        });
    }
}

As you can see above, we have moved all the configuration for the Student entity into constructor of StudentEntityConfiguration, which is derived from EntityTypeConfiguration<Student>. You need to specify entity type in a generic place holder for which you include configurations, Student in this case.

Now, you can inform Fluent API about this class, as shown below.

public class SchoolDBContext: DbContext
{
    public SchoolDBContext(): base()
    {
    }

    public DbSet<Student> Students { get; set; }
    public DbSet<Standard> Standards { get; set; }
    public DbSet<StudentAddress> StudentAddress { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
            // Moved all Student related configuration to StudentEntityConfiguration class
            modelBuilder.Configurations.Add(new StudentEntityConfiguration());

    }
}

Thus, you can use a separate class to configure a domain class to increase the readability and maintainability.