Understanding Entity Relationships in C#: One-to-One, One-to-Many, and Many-to-Many Explained

We have different types of Entity Relationships:

1. One to One

2. One to Many

3. Many to Many

One-to-One Relationship

Example: A person has one passport, and a passport is assigned to one person.

Entities:

• Person

• Passport

Schema:

public class Person
{
    public int PersonId { get; set; }
    public string Name { get; set; } = string.Empty;

    // Navigation property
    public Passport Passport { get; set; }
}

public class Passport
{
    public int PassportId { get; set; }
    public string Number { get; set; } = string.Empty;

    public int PersonId { get; set; }

    // Navigation property
    public Person Person { get; set; }
}

Fluent API Configuration:

modelBuilder.Entity<Person>()
    .HasOne(p => p.Passport)
    .WithOne(p => p.Person)
    .HasForeignKey<Passport>(p => p.PersonId);

One-to-Many Relationship

Example: A teacher can have many students, but a student can only have one teacher.

Entities:

• Teacher

• Student

Schema:

public class Teacher
{
    public int TeacherId { get; set; }
    public string Name { get; set; } = string.Empty;

    // Navigation property
    public ICollection<Student> Students { get; set; } = new List<Student>();
}

public class Student
{
    public int StudentId { get; set; }
    public string Name { get; set; } = string.Empty;

    public int TeacherId { get; set; }

    // Navigation property
    public Teacher Teacher { get; set; }
}

Fluent API Configuration:

modelBuilder.Entity<Teacher>()
    .HasMany(t => t.Students)
    .WithOne(s => s.Teacher)
    .HasForeignKey(s => s.TeacherId);

Many-to-Many Relationship

Example: A student can enroll in many courses, and a course can have many students.

Entities:

• Student

• Course

• Enrollment (joining table)

Schema:

public class Student
{
    public int StudentId { get; set; }
    public string Name { get; set; } = string.Empty;

    // Navigation property
    public ICollection<Enrollment> Enrollments { get; set; } = new List<Enrollment>();
}

public class Course
{
    public int CourseId { get; set; }
    public string Title { get; set; } = string.Empty;

    // Navigation property
    public ICollection<Enrollment> Enrollments { get; set; } = new List<Enrollment>();
}

public class Enrollment
{
    public int StudentId { get; set; }
    public Student Student { get; set; }

    public int CourseId { get; set; }
    public Course Course { get; set; }
}

Fluent API Configuration:

modelBuilder.Entity<Enrollment>()
    .HasKey(e => new { e.StudentId, e.CourseId });

modelBuilder.Entity<Enrollment>()
    .HasOne(e => e.Student)
    .WithMany(s => s.Enrollments)
    .HasForeignKey(e => e.StudentId);

modelBuilder.Entity<Enrollment>()
    .HasOne(e => e.Course)
    .WithMany(c => c.Enrollments)
    .HasForeignKey(e => e.CourseId);