How to Implement Database Relationships in Entity Framework

 Implementing database relationships in Entity Framework (EF) allows you to define how different tables (entities) relate to each other. This is essential for building structured, relational data models in .NET applications.

✅ Types of Relationships in EF Core

One-to-One

One-to-Many

Many-to-Many

All relationships in EF are defined through:

Navigation properties

Foreign keys

Fluent API or Data Annotations

🧱 1. One-to-Many Relationship Example

(Example: One Author → Many Books)

✅ Models:

csharp

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public class Author

{

    public int AuthorId { get; set; }

    public string Name { get; set; }

    public List<Book> Books { get; set; } // Navigation property

}

public class Book

{

    public int BookId { get; set; }

    public string Title { get; set; }

    public int AuthorId { get; set; }       // Foreign Key

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

}

🔧 Fluent API (optional but more control):

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protected override void OnModelCreating(ModelBuilder modelBuilder)

{

    modelBuilder.Entity<Book>()

        .HasOne(b => b.Author)

        .WithMany(a => a.Books)

        .HasForeignKey(b => b.AuthorId);

}

🧱 2. One-to-One Relationship

(Example: One User → One UserProfile)

✅ Models:

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public class User

{

    public int UserId { get; set; }

    public string Email { get; set; }

    public UserProfile Profile { get; set; }

}

public class UserProfile

{

    public int UserProfileId { get; set; }

    public string Bio { get; set; }

    public int UserId { get; set; }

    public User User { get; set; }

}

🔧 Fluent API:

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protected override void OnModelCreating(ModelBuilder modelBuilder)

{

    modelBuilder.Entity<User>()

        .HasOne(u => u.Profile)

        .WithOne(p => p.User)

        .HasForeignKey<UserProfile>(p => p.UserId);

}

🧱 3. Many-to-Many Relationship

(Example: Students ↔ Courses)

✅ EF Core 5+ supports skip navigations (no join entity needed):

csharp

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public class Student

{

    public int StudentId { get; set; }

    public string Name { get; set; }

    public List<Course> Courses { get; set; }

}

public class Course

{

    public int CourseId { get; set; }

    public string Title { get; set; }

    public List<Student> Students { get; set; }

}

EF will automatically generate a join table (e.g., StudentCourse).

📦 Entity Framework Setup Example

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dotnet add package Microsoft.EntityFrameworkCore

dotnet add package Microsoft.EntityFrameworkCore.SqlServer

dotnet add package Microsoft.EntityFrameworkCore.Tools

🔧 DbContext Example

csharp

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public class SchoolContext : DbContext

{

    public DbSet<Student> Students { get; set; }

    public DbSet<Course> Courses { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)

        => options.UseSqlServer("YourConnectionStringHere");

}

🛠️ Tools

Use dotnet ef migrations add InitialCreate to create migration

Use dotnet ef database update to apply changes to your DB

✅ Summary Table

Relationship Type Use Cases Method

One-to-One User ↔ Profile HasOne().WithOne()

One-to-Many Author → Books HasOne().WithMany()

Many-to-Many Students ↔ Courses Use navigation lists only (EF Core 5+)

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Read More

Database Migrations in Entity Framework Core

Entity Framework Core: The ORM for Full Stack .NET Developers

Working with NoSQL Databases in .NET (MongoDB, Redis)

Setting Up a SQL Server Database in Full Stack .NET Applications

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