🪴 Shoyeb's Dev Diary

8.Entity Framework and Seed

Thursday Mar 13, 2025

We can use the entity framework as two ways:

  1. Code First
  2. Database First

1. Database First

Way 1: Just go to database > select design > properties > apply relation.      Now come on web project > create a folder (Name like EF) > Right click on folder and select edmx

Way 2: Install nuget Microsoft.EntityFrameworkCore.SqlServer and Microsoft.EntityFrameworkCore.Tools

 
Scaffold-DbContext "Server=.\MSSQLSERVER19"; Database=TempoDB; User Id = sa; Password=123456;" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models -Context TempoDbContext
 
or,
 
Scaffold-DbContext "Server=.\MSSQLSERVER19"; Database=TempoDB; User Id = sa; Password=123456;" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models

2. Code First

Way 1: Migration Command for Dotnet Framework:

Gererate migration:           enable-migrations         and then           add-migration AllTableCreation

Delete last migration:        remove-migration

Update to database:           update-database –Verbose

Way 2: Migration Command for Core:

Install:      dotnet tool install —global dotnet-ef
Uninstall:    dotnet tool uninstall —global dotnet-ef
Update:       dotnet tool update —global dotnet-ef

Gererate migration:           dotnet ef migrations add CreateStudentTable —project University.Web —context ApplicationDbContext

Delete last migration:        dotnet ef migrations remove —project University.Web —context ApplicationDbContext

Update to database:           dotnet ef database update —project University.Web —context ApplicationDbContext

Example: EF Core Code First Migration

Step 1: Using Data Annotation Attributes

  1. Setup:

    • Create a Console Project.
    • Install NuGet packages:
      • Microsoft.EntityFrameworkCore
      • Microsoft.EntityFrameworkCore.SqlServer
      • Microsoft.EntityFrameworkCore.Design
    • Define entity classes (Teacher, Course, Student, etc.).
    • Implement MyDbContext to define DbSets and configure database connection.

  2. Code Details:

    • Use Data Annotations like [Key], [Required], [ForeignKey] to define relationships and constraints.

Classes:

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
 
public class Teacher
{
    [Key]
    public int Id { get; set; }
    
    [Required]
    public string Name { get; set; }
    
    public virtual ICollection<Course> Courses { get; set; }
 
    public Teacher()
    {
        Courses = new List<Course>();
    }
}
 
public class Course
{
    [Key]
    public int Id { get; set; }
    
    [Required]
    public string Title { get; set; }
    
    [Required]
    public double Fees { get; set; }
    
    [ForeignKey("Teacher")]
    public int TeacherId { get; set; }
    
    public virtual Teacher Teacher { get; set; }
    
    public virtual ICollection<CourseStudent> CourseStudents { get; set; }
 
    public Course()
    {
        CourseStudents = new List<CourseStudent>();
    }
}
 
public class CourseStudent
{
    [Key]
    public int Id { get; set; }
    
    [ForeignKey("Course")]
    public int CourseId { get; set; }
    
    [ForeignKey("Student")]
    public int StudentId { get; set; }
    
    public virtual Course Course { get; set; }
    public virtual Student Student { get; set; }
    
    public DateTime EnrollmentDate { get; set; }
}
 
public class Student
{
    [Key]
    public int Id { get; set; }
    
    [Required]
    public string Name { get; set; }
    
    [Required]
    public int Roll { get; set; }
    
    [ForeignKey("Mentor")]
    public int MentorId { get; set; }
    
    public virtual Mentor Mentor { get; set; }
    
    public virtual ICollection<CourseStudent> CourseStudents { get; set; }
 
    public Student()
    {
        CourseStudents = new List<CourseStudent>();
    }
}
 
public class Mentor
{
    [Key]
    public int Id { get; set; }
    
    [Required]
    public string Name { get; set; }
}

DbContext Example:

public class MyDbContext : DbContext
{
    private readonly string _connectionString;
    private readonly string _migrationAssembly;
 
    public MyDbContext()
    {
        _connectionString = @"server = .\SQLEXPRESS; Database= TestDB; Trusted_Connection=True; TrustServerCertificate=True;";
        _migrationAssembly = Assembly.GetExecutingAssembly().GetName().Name;
    }
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            optionsBuilder.UseSqlServer(_connectionString, (x) => x.MigrationsAssembly(_migrationAssembly));
        }
        base.OnConfiguring(optionsBuilder);
    }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
    }
    public DbSet<Teacher> Teachers { get; set; }
    public DbSet<Course> Courses { get; set; }
    public DbSet<CourseStudent> CourseStudents { get; set; }
    public DbSet<Student> Students { get; set; }
    public DbSet<Mentor> Mentors { get; set; }
}

Program.cs:

//.............Insert..........
using (var context = new MyDbContext())
{
    var teacher = new Teacher { Name = "Alice" };
    var course = new Course { Title = "Math 101", Fees = 100.0 };
    var student = new Student { Name = "Bee", Roll = 124 };
    var mentor = new Mentor { Name = "Prof. Lee" };
 
    // Set up relationships
    course.Teacher = teacher;
    student.Mentor = mentor;
    var courseStudent = new CourseStudent
    {
        Course = course,
        Student = student,
        EnrollmentDate = DateTime.Now
    };
 
    context.Teachers.Add(teacher);
    context.Courses.Add(course);
    context.Students.Add(student);
    context.Mentors.Add(mentor);
    context.CourseStudents.Add(courseStudent);
    context.SaveChanges();
}
 
//............Edit.....................
using (var context = new MyDbContext())
{
    // Find course title where teacher included
    var courseToUpdate = context.Courses
        .Include(c => c.Teacher)
        .FirstOrDefault(c => c.Title == "Math 101");
    // Find student name where mentor included
    var studentToUpdate = context.Students
        .Include(s => s.Mentor)
        .FirstOrDefault(s => s.Name == "Bee");
 
    // Check if the entities were found
    if (courseToUpdate != null && studentToUpdate != null)
    {
        // Update the entities
        courseToUpdate.Title = "Physics";
        studentToUpdate.Name = "Diana";
        courseToUpdate.Teacher.Name = "Prof Zing";
        studentToUpdate.Mentor.Name = "Maria";
        context.SaveChanges();
    }
}
 
//.........Delete By Id.....
using (var context = new MyDbContext())
{
    var entityToDelete = context.CourseStudents.Find(4);
    if (entityToDelete != null)
    {
        context.CourseStudents.Remove(entityToDelete);
        context.SaveChanges();
    }
}

CRUD Operations:

  • Insert: Add and save entities with relationships.
  • Edit: Update entities and their related objects.
  • Delete: Remove entities using their IDs.

Step 2: Using Fluent API

  1. Setup:
    • Same NuGet packages and structure as Step 1.
    • Use Fluent API in OnModelCreating for defining relationships and constraints.

Classes:

 
public class Teacher
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<Course> Courses { get; set; }
}
 
public class Course
{
    public int Id { get; set; }
    public string Title { get; set; }
    public double Fees { get; set; }
    public int TeacherId { get; set; }
    public Teacher Teacher { get; set; }
    public ICollection<CourseStudent> CourseStudents { get; set; }
}
 
public class CourseStudent
{
    public int CourseId { get; set; }
    public Course Course { get; set; }
    public int StudentId { get; set; }
    public Student Student { get; set; }
    public DateTime EnrollmentDate { get; set; }
}
 
public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Roll { get; set; }
    public int MentorId { get; set; }
    public Mentor Mentor { get; set; }
    public virtual ICollection<CourseStudent> CourseStudents { get; set; }
}
 
public class Mentor
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Key Configuration in OnModelCreating:

public class MyDbContext : DbContext
{
    private readonly string _connectionString;
    private readonly string _migrationAssembly;
 
    public MyDbContext()
    {
        _connectionString = @"server = .\SQLEXPRESS; Database= TestDB; Trusted_Connection=True; TrustServerCertificate=True;";
        _migrationAssembly = Assembly.GetExecutingAssembly().GetName().Name;
    }
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            optionsBuilder.UseSqlServer(_connectionString, (x) => x.MigrationsAssembly(_migrationAssembly));
        }
        base.OnConfiguring(optionsBuilder);
    }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
 
        modelBuilder.Entity<CourseStudent>()
            .HasKey(x => new { x.CourseId, x.StudentId });
 
        modelBuilder.Entity<CourseStudent>()
            .HasOne(x => x.Course)
            .WithMany(x => x.CourseStudents)
            .HasForeignKey(x => x.CourseId);
 
        modelBuilder.Entity<CourseStudent>()
            .HasOne(x => x.Student)
            .WithMany(x => x.CourseStudents)
            .HasForeignKey(x => x.StudentId);
 
        modelBuilder.Entity<Course>()
            .HasOne(x => x.Teacher)
            .WithMany(c => c.Courses)
            .HasForeignKey(f => f.TeacherId);
 
        modelBuilder.Entity<Teacher>()
            .HasMany(x => x.Courses)
            .WithOne(c => c.Teacher);
 
        modelBuilder.Entity<Student>()
            .HasOne(x => x.Mentor)
            .WithOne()
            .HasForeignKey<Student>(f => f.MentorId);
    }
    public DbSet<Teacher> Teachers { get; set; }
    public DbSet<Course> Courses { get; set; }
    public DbSet<CourseStudent> CourseStudents { get; set; }
    public DbSet<Student> Students { get; set; }
    public DbSet<Mentor> Mentors { get; set; }
}

Program.cs:

//............Insert......................
using (var context = new MyDbContext())
{
    var teacher = new Teacher { Name = "Alice" };
    var course = new Course { Title = "Math 101", Fees = 100.0 };
    var student = new Student { Name = "Bee", Roll = 124 };
    var mentor = new Mentor { Name = "Prof. Lee" };
 
    // Set up relationships
    course.Teacher = teacher;
    student.Mentor = mentor;
    var courseStudent = new CourseStudent
    {
        Course = course,
        Student = student,
        EnrollmentDate = DateTime.Now
    };
 
    context.Teachers.Add(teacher);
    context.Courses.Add(course);
    context.Students.Add(student);
    context.Mentors.Add(mentor);
    context.CourseStudents.Add(courseStudent);
    context.SaveChanges();
}
 
//.............Edit...................
using (var context = new MyDbContext())
{
    // Find course title where teacher included
    var courseToUpdate = context.Courses
        .Include(c => c.Teacher)
        .FirstOrDefault(c => c.Title == "Math 101");
    // Find student name where mentor included
    var studentToUpdate = context.Students
        .Include(s => s.Mentor)
        .FirstOrDefault(s => s.Name == "Bee");
 
    // Check if the entities were found
    if (courseToUpdate != null && studentToUpdate != null)
    {
        // Update the entities
        courseToUpdate.Title = "Physics";
        studentToUpdate.Name = "Alisa";
        courseToUpdate.Teacher.Name = "Prof Zing";
        studentToUpdate.Mentor.Name = "Maria";
        context.SaveChanges();
    }
}
 
//Delete Courses and Related table data.........
using (var context = new MyDbContext())
{
    var entityToDelete = context.Courses.Find(1);
    if (entityToDelete != null)
    {
        context.Courses.Remove(entityToDelete);
        context.SaveChanges();
    }
}

Data Seeding

EF Core provides two ways to seed data:

  1. Separate Class (Recommended): Use a dedicated seeding class for better organization. Refer to EF Core Seeding Documentation.

  2. Inline in OnModelCreating:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);
 
    for (int i = 20; i <= 30; i++)
    {
        modelBuilder.Entity<Teacher>().HasData(new Teacher { Id = i, Name = $"Teacher {i}" });
    }
}

Important Notes:

  • For entities with foreign keys, ensure the referenced entities exist to avoid errors.
  • Avoid using loops to seed data for one-to-one relationships, as they may lead to conflicts.