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Lesson 6020 min read

EF Core Relationships

Learn how to model one-to-many and many-to-many relationships between entities in Entity Framework Core.

One-to-Many Relationships

EF Core infers relationships from your entity classes by convention — a foreign key property and a navigation property are usually all you need.

public class Project
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public List<TaskItem> Tasks { get; set; } = new(); // one Project has many Tasks
}


public class TaskItem
{
    public int Id { get; set; }
    public string Title { get; set; } = "";


    public int ProjectId { get; set; }       // foreign key
    public Project? Project { get; set; }    // navigation property
}

By default, EF Core doesn't load related data automatically — you opt in with `Include`, which prevents accidentally fetching far more data than a query actually needs.

List<Project> projects = await context.Projects
    .Include(p => p.Tasks)
    .ToListAsync();


foreach (var project in projects)
{
    Console.WriteLine($"{project.Name}: {project.Tasks.Count} tasks");
}

Many-to-Many Relationships

Modern EF Core can configure many-to-many relationships directly, without you needing to define the join table entity yourself.

public class TaskItem
{
    public int Id { get; set; }
    public string Title { get; set; } = "";
    public List<Tag> Tags { get; set; } = new();
}


public class Tag
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public List<TaskItem> Tasks { get; set; } = new();
}
// EF Core automatically creates a hidden join table for this

FAQs

The navigation property will simply be empty/null rather than throwing an error — a common source of confusing bugs, so it's worth checking for whenever related data seems to be missing.

Summary

  • EF Core infers relationships from foreign keys and navigation properties.
  • Include() explicitly loads related data — it isn't loaded automatically.
  • Modern EF Core configures many-to-many relationships without a manual join entity.
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Dependency Injection