EF Core Relationships — One-to-One, One-to-Many, Many-to-Many

Introduction

ShopNest Academy (School Management) connects Students, Courses, and Teachers through EF Core relationships. Modeling them correctly avoids duplicate data, orphaned records, and slow queries.

After this article you will

• Configure one-to-one, one-to-many, and many-to-many
• Use navigation properties both directions
• Apply Fluent API for cascade delete rules
• Choose eager, lazy, or explicit loading
• Detect and fix N+1 query problems

Prerequisites

• Article 16 — EF Core LINQ Queries
• ShopNest DbContext with Products/Orders from Articles 13–15
• SQL Server or LocalDB configured

Concept deep-dive

One-to-one

Student → StudentProfile (one profile per student). FK on dependent side.

public class Student
{
    public int Id { get; set; }
    public StudentProfile Profile { get; set; }
}
public class StudentProfile
{
    public int Id { get; set; }
    public int StudentId { get; set; }
    public Student Student { get; set; }
}

One-to-many

Teacher → many Courses. FK TeacherId on Course.

Many-to-many (.NET 6+)

Students ↔ Courses via implicit join table, or explicit Enrollment entity with Grade and EnrolledAt.

public class Enrollment
{
    public int StudentId { get; set; }
    public Student Student { get; set; }
    public int CourseId { get; set; }
    public Course Course { get; set; }
    public decimal? Grade { get; set; }
}

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

Cascade delete

OnDelete(DeleteBehavior.Cascade) — deleting Teacher deletes Courses? Usually Restrict on business FKs to prevent accidents. Cascade OK for owned child rows (OrderLines).

Loading strategies

Hands-on — ShopNest School Management System

1. Scaffold Student, Course, Teacher, Enrollment entities.
2. Migration with FK constraints and Restrict on Teacher→Course.
3. Student details page: Include Profile + Enrollments.ThenInclude(Course).
4. Log SQL — verify single query vs N+1.

Common errors & best practices

• Multiple cascade paths in SQL Server — use Restrict on one relationship.
• Missing inverse navigation — EF can still work but Fluent config is clearer.
• Many-to-many without join entity when you need extra columns on enrollment.

Interview questions

Q: N+1 problem?
A: 1 query for list + N queries for related data — fix with Include or projection.

Q: When explicit join entity?
A: When relationship has payload (Grade, EnrolledAt) or audit fields.

Q: Cascade delete safe?
A: Rarely on master data; OK for dependent detail rows owned by parent.

Summary

• Model relationships with navigation properties + FKs
• Fluent API controls cascade and join tables
• Include eagerly; avoid lazy loading in ASP.NET request paths
• Enrollment join entity for Student-Course grades

Previous: EF Core LINQ Queries
Next: EF Core Fluent API

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