EF Core Linqkit Use Expressions in EntitySets or EntityCollections

By default, EntitySet<> or EntityCollection<> does not implements IQueryable<>. It means that we can't call methods that accept an Expression<Func<>> such as, Queryable's Any, and our query won't compile.

So, let's suppose we want to retrieve all those authors' names that had written books of more than 1000 pages, or you can say that we will write a method and pass the page criteria predicate as a parameter as shown below.

public static List<string> GetAuthorsExample1(Expression<Func<Book, bool>> pageCriteria)
{
    using (var context = new BookStore())
    {
        var query = context.Authors
            //.Where(a => a.Books.Any(pageCriteria)) // not compiling
            .Select(a => a.Name);

        return query.ToList();
    }
}

Now to make it works, LinqKit.Microsoft.EntityFrameworkCore allows you to call AsExpandable() on the Table<> object and also call Compile() on the expression variable, when used on an EntitySet or EntityCollection as shown below.

public static List<string> GetAuthorsExample2(Expression<Func<Book, bool>> pageCriteria)
{
    using (var context = new BookStore())
    {
        var query = context.Authors.AsExpandable()
            .Include(a => a.Books)
            .Where(a => a.Books.Any(pageCriteria.Compile()))
            .Select(a => a.Name);

        return query.ToList();
    }
}

You can call the above method and pass the page criteria as shown below.

var authors = ExpressionsInEntitySetsOrEntityCollections.GetAuthorsExample2(b => b.NoOfPages > 1000);
foreach (var author in authors)
{
    Console.WriteLine(author);
}

Let's run the above code, and you will see the following output.

Carson Alexander
Arturo Anand