How to reference an entity from different entity types in EF Core - entity-framework-core

Question

In my application I need to model the concept of a Link. It contains a name, a description and of course, a URL.
For the sake of simplicity, let´s assume I have 2 entities that can have many links: companies and projects.
If I add an ICollection of Link (generics) to the Company entity, EF will add a FK from Link to Company in the Link table. The same will happen if I add an ICollection of Link to the project entity.
I want the Link table to be completely agnostic of "who" might be referencing its records, and at the same time, being able to reference Link objects from wherever they are needed.
Is that possible at all?
Thanks!

Answer 1

The only way I know is through the use of a Principal Key. I'll use Link and Company in this example. The models as you indicated:
public class Link
{
public int Id { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public string URL { get; set; }
public string CompanyName { get; set; }
}
I added CompanyName, more on that later.
public class Company
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<Link> Links { get; set; }
}
So as you may have discovered, even if there's no integer FK property explicitly defined in the child class, EF Core will still create it in the database anyway.
It's not really possible (as far as I know) to have a table that is completely unaware of its relationship with others. However, with principal keys you can set the FK to reference a property in the parent class other than the primary key, and for that purpose I added CompanyName to Link. It will reference the Name property in Company and use that to fill in its value. Here's the mapping you need to do in the DbContext class:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Link>()
.HasOne<Company>()
.WithMany(c => c.Links)
.HasForeignKey(l => l.CompanyName)
.HasPrincipalKey(c => c.Name);
}
Here's the resulting database schema after applying the migration (SQL Server):
This is the data I used to test it out:
var newCompanies = new List<Company>()
{
new Company()
{
Name = "EF Core Enterprises",
Links = new List<Link>()
{
new Link()
{
Name = "Introduction",
Description = "The EF Core 101",
URL = "https://efcore.com/intro"
},
new Link()
{
Name = "Mapping",
Description = "Mapping classes to database objects",
URL = "https://efcore.com/mapping"
},
new Link()
{
Name = "Change Tracker",
Description = "Deep dive into EF Core's object tracking mechanism",
URL = "https://efcore.com/changetracker"
}
}
},
new Company()
{
Name = "ASP.NET Core Inc.",
Links = new List<Link>()
{
new Link()
{
Name = "Authentication",
Description = "Authentication Basics",
URL = "https://asp-net-core.com/auth"
},
new Link()
{
Name = "Routing",
Description = "Configure routing for web apps",
URL = "https://asp-net-core.com/routing"
},
new Link()
{
Name = "API",
Description = "Create a REST API with ASP.NET Core",
URL = "https://asp-net-core.com/API"
}
}
}
};
context.Companies.AddRange(newCompanies);
context.SaveChanges();
So, is it possible to eager load the Links for a company with Include()? Yes, nothing is different.
var companies = context.Companies
.Include(c => c.Links)
.ToList();
foreach (var company in companies)
{
foreach (var link in company.Links)
{
Console.WriteLine($"{link.CompanyName} --> {link.Name} at {link.URL}");
}
}
And the output:
EF Core Enterprises --> Introduction at https://efcore.com/intro
EF Core Enterprises --> Mapping at https://efcore.com/mapping
EF Core Enterprises --> Change Tracker at https://efcore.com/changetracker
ASP.NET Core Inc. --> Authentication at https://asp-net-core.com/auth
ASP.NET Core Inc. --> Routing at https://asp-net-core.com/routing
ASP.NET Core Inc. --> API at https://asp-net-core.com/API
Sorry for being too lengthy, I just like to explain everything as clearly as possible. Here's the documentation concerning relationships. I hope you find this useful.