Question

# Question

I have a concern like this: in the database, there will be no distinct tables for passengers, drivers or managers, but only a `users` table. therefore there should only be User entity (EF Core) to map to the table. however, the `users` table has one-to-many relationship with `vehicles`, and this should only apply to users that have role `driver` (Role is an entity, representing the `roles` table). How should I handle this?

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# Answer

This is a common issue in **EF Core when using a single "Users" table for multiple roles** while enforcing  **role-based constraints** . Since **only drivers** should have a `vehicles` relationship, you have a few design options:

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## **Option 1: Use Navigation Property with Conditional Loading**

You  **keep a single `User` entity** , but filter vehicle access  **based on role** .

### **Implementation**

```csharp
public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Role Role { get; set; }  // Navigation property to Role
    public int RoleId { get; set; } // Foreign Key

    // Navigation property: Only relevant for Drivers
    public List<Vehicle>? Vehicles { get; set; }
}
```

```csharp
public class Vehicle
{
    public int Id { get; set; }
    public string LicensePlate { get; set; }
    public int UserId { get; set; } // FK to User
    public User User { get; set; }  // Navigation property
}
```

### **Enforcing the Role Restriction**

You **filter access to vehicles in queries** (prevent non-drivers from having vehicles):

```csharp
var driversWithVehicles = dbContext.Users
    .Where(u => u.Role.Name == "Driver")
    .Include(u => u.Vehicles)
    .ToList();
```

📌  **Pros** : Keeps database simple, avoids unnecessary table inheritance.

📌  **Cons** : Need to **manually enforce role-based constraints** in logic.

---

## **Option 2: Use Backing Fields with Explicit Role Checks**

Instead of exposing `Vehicles` to all users, **use a private backing field** to ensure only drivers have access.

### **Implementation**

```csharp
public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Role Role { get; set; }
    public int RoleId { get; set; }

    // Private field (EF Core still maps this correctly)
    private List<Vehicle>? _vehicles;

    // Public property ensures only Drivers can access
    public IReadOnlyList<Vehicle>? Vehicles
        => Role?.Name == "Driver" ? _vehicles : null;
}
```

📌  **Pros** : Prevents incorrect usage at the code level.

📌  **Cons** : Requires manual enforcement at **object level** (EF Core does not enforce this automatically).

---

## **Option 3: Table Splitting (Single Table with Different Mappings)**

EF Core **table splitting** allows mapping a subset of properties  **only when a condition is met** .

### **Implementation**

```csharp
public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Role Role { get; set; }
}

public class Driver
{
    public int Id { get; set; } // Same as User Id
    public List<Vehicle> Vehicles { get; set; }
}
```

### **EF Core Fluent API Mapping**

```csharp
modelBuilder.Entity<User>()
    .ToTable("Users");

modelBuilder.Entity<Driver>()
    .ToTable("Users") // Same table
    .HasOne<Role>()
    .WithMany()
    .HasForeignKey(u => u.Id)
    .HasPrincipalKey(u => u.Id);
```

📌  **Pros** : EF Core ensures **only drivers** have vehicles automatically.

📌  **Cons** : More complex EF Core configuration.

---

## **Which Approach is Best?**

| Approach                         | Simplicity | Enforces Role Constraint | Works Well with EF Core |
| -------------------------------- | ---------- | ------------------------ | ----------------------- |
| **1: Conditional Loading** | ✅ Simple  | ❌ Manual enforcement    | ✅ Good for performance |
| **2: Backing Fields**      | ✅ Simple  | ✅ Enforces constraint   | ✅ Works well           |
| **3: Table Splitting**     | ❌ Complex | ✅ Automatic enforcement | ✅ EF Core manages well |

---

## **📌 Recommendation**

* **If you want a simple solution** → Use **Option 1 (Conditional Loading)**
* **If you want stricter role constraints** → Use **Option 2 (Backing Fields)**
* **If EF Core should enforce the structure automatically** → Use **Option 3 (Table Splitting)**

Do you want help implementing one of these fully in  **Fluent API** ? 🚀

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# References

- [Navigation property](https://learn.microsoft.com/vi-vn/dotnet/framework/data/adonet/navigation-property)