在EF Core中维护派生类型中的关系

Question

I am facing some problem to maintain the relationship in derived types and query them using LINQ. Please consider the following scenario. Let's say I have the below hierarchy:

public class Company
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public abstract class Document
{
    public int Id { get; set; }
    public string DocType { get; set; }
    public string Name { get; set; }
}

public class CompanyDoc : Document
{
    public int CompanyId { get; set; }
    public Company Company { get; set; }
}

public class PersonDoc : Document
{
    public int PersonId { get; set; }
    public Person Person { get; set; }
}

That means I have a document object and the owner of the document can be any company or any person so I create 2 derived document company doc & person doc.

My question is, is it ok to maintain a relationship like this way, if not then what is the best approach to maintain this kind of hierarchy?

In ef core 2.1 I can handle this hierarchy using TPH. But If I want to get all the document along with the owner what will be the linq query. I have tried with below one but it does not work.

var doc = (from d in _context.Set<Document>()
            join c in _context.Company on (d as CompanyDoc).CompanyId equals c.Id into cd
            from cdoc in cd.DefaultIfEmpty()
            join c in _context.Person on (d as PersonDoc).PersonId equals c.Id into pd
            from pdoc in pd.DefaultIfEmpty()
            select new {
                d.Id,
                d.Name,
                Owner = cdoc.Name != null ? cdoc.Name : pdoc.Name
            }).ToList()

Can you please help me by sharing your thoughts. Please take this as a hypothetical example.

Answer 1

I think you are overcomplicating the query. EF Core is quite clever on how it handles Table per hierarchy (TPH) queries. By using the abstract class as the DbSet<T> property you can access all the types, and filter out what you want.

So, in your example the DbContext would contain a property public DbSet<Document> Documents { get ; set; } public DbSet<Document> Documents { get ; set; } public DbSet<Document> Documents { get ; set; } and the following queries would work.

//This returns all the documents - each document is of the correct type
//e.g PersonDocument or CompanyDocument
var allDocs = context.Documents.ToList();

//This would only return PersonDocuments - change the type for other versions
var personDocs = context.Documents.OfType<PersonDocument>().ToList();

EF Core does all the hard work to get the correct data and type for you. It's pretty nice to use.

PS. If you have my book Entity Framework Core in Action then I cover TPH in section 7.8.2, especially page 201.