EF, Repositories and crossing aggregate boundaries

Question

I have a two aggregate roots in my domain, and therefore two repositories. We'll call them BookRepository, and AuthorRepository, for the sake of example.

I'm designing an MVC application, and one page has to display a table containing a list of authors, with each row showing the author's personal details. At the end of each row is a small button that can be clicked to expand the row and show a child table detailing the author's published books.

When the page loads, some ajax is executed to retrieve the Author details from an API controller and display the data in the table. Each property in an Author object maps almost directly to a column, with one exception, and this is where I'm having my problem. I want the button at the end of each row to be disabled, if and only if the author has no published books. This means that a boolean has to returned with each Author record, indicating if they have any published books.

My book repository has a couple of methods like this:

public IEnumerable<Book> GetBooksForAuthor(int authorId);

public bool AnyBooksForAuthor(int authorId);

and my Book class has a property called AuthorId, so I can retrieve a book's author by calling

authorRepository.GetById(book.AuthorId);

My problem is that in order to create a row for my aforementioned table, I need to create it like this:

IEnumerable<Author> authors = authorRepository.GetAll();
foreach (Author author in authors)
{
    yield return new AuthorTableRow
    {
        Name = author.Name,
        Age = author.Age,
        Location = author.PlaceOfResidence.Name,
        HasBooks = this.bookRepository.AnyBooksForAuthor(author.Id)
    };
}

The above code seems correct, but there's a fairly heft performance penalty in calling this.bookRepository.AnyBooksForAuthor(author.Id) for every single author, because it performs a database call each time.

Ideally, I suppose I would want an AuthorTableRowRepository which could perform something like the following:

public IEnumerable<AuthorTableRow> GetAll()
{
    return from a in this.dbContext.Authors
           select new AuthorTableRow
           {
               Name = a.Name,
               Age = a.Age,
               Location a.PlaceOfResidence.Name
               HasBooks = a.Books.Any()
           });
}

I'm hesitant to put this in place for these reasons :

AuthorTableRowRepository is a repository of AuthorTableRows, but AuthorTable row is not a domain object, nor an aggregate root, and therefore should not have its own repository.
As Author and Book are both aggregate roots, I removed the "Books" property from the Author entity, because I wanted the only way to retrieve books to be via the BookRepository. This makes HasBooks = a.Books.Any() impossible. I am unsure whether I am imposing my own misguided best practice here though. It seems wrong to obtain Books by obtaining an Author via the AuthorRepository and then going through its Books property, and vice versa in obtaining an Author via a property on a Book object. Crossing aggregate root boundaries would be the way I'd term it, I suppose?

How would other people solve this? Are my concerns unfounded? I am mostly concerned about the (what should be a) performance hit in the first method, but I want to adhere to best practice with the Repository pattern and DDD.

Why downvotes...? Please explain so I can avoid any mistakes next time? — dark_perfect

Tom Van Acker Tom Van Acker · Accepted Answer · 2014-09-22T09:53:26

I would stick to the first approach, but try to optimize things in the bookrepository method. For instance, you can load this information all in one time, and use in-memory lookup to speed this up. Like this you would need 2 queries, and not 1 for each author.

EF, Repositories and crossing aggregate boundaries

2 Answers