EF Core: One-to-One vs. One-to-Many (A Painful Lesson)
The difference between a One-to-One and a One-to-Many relationship in Entity Framework Core isn’t just academic. On my last project, choosing the wrong one led to two weeks of painful migrations, late nights, and a whole lot of dotnet ef database update anxiety.
It started with a simple feature: each tenant in our multi-tenant app needed a settings object. A Tenant has one Settings. Simple. So, we modeled it as a One-to-One relationship. What could go wrong?
Six months later, the requirements changed. Now, the business wanted an audit trail. They needed to see who changed a setting and when. Suddenly, our single Settings record per tenant was a huge problem. We needed a history, a collection of settings over time. Our rigid One-to-One schema was now a roadblock, and I was the one who had to fix it.
This experience taught me a hard lesson: your data model has to account for the future, not just the current sprint.
The Basics: What’s the Actual Difference?
Most of us get the concept, but let’s quickly recap how EF Core sees them.
A One-to-One relationship is a tight coupling. Think User and UserProfile. A user has exactly one profile. The profile can’t exist without the user. In the database, the primary key of UserProfile is also its foreign key back to User.
// The principal entity
public class User
{
public int Id { get; set; }
public string Username { get; set; }
// Navigation property to the dependent
public UserProfile Profile { …