使用联接防止Entity Framework查询中的SELECT N + 1问题

Question

I'm trying to query something from an indirectly related entity into a single-purpose view model. Here's a repro of my entities:

public class Team {
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Member> Members { get; set; }
}

public class Member {
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Pet {
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    public Member Member { get; set; }
}

Each class is in a DbSet<T> in my database context.

This is the view model I want to construct from a query:

public class PetViewModel {
    public string Name { get; set; }
    public string TeamItIndirectlyBelongsTo { get; set; }
}

I do so with this query:

public PetViewModel[] QueryPetViewModel_1(string pattern) {
    using (var context = new MyDbContext(connectionString)) {
        return context.Pets
            .Where(p => p.Name.Contains(pattern))
            .ToArray()
            .Select(p => new PetViewModel {
                Name = p.Name,
                TeamItIndirectlyBelongsTo = "TODO",
            })
            .ToArray();
    }
}

But obviously there's still a "TODO" in there.

Gotcha: I can not change the entities at this moment , so I can't just include a List<Pet> property or a Team property on Member to help out. I want to fix things inside the query at the moment.

Here's my current solution:

public PetViewModel[] QueryPetViewModel_2(string pattern) {
    using (var context = new MyDbContext(connectionString)) {
        var petInfos = context.Pets
            .Where(p => p.Name.Contains(pattern))
            .Join(context.Members,
                p => p.Member.Id,
                m => m.Id,
                (p, m) => new { Pet = p, Member = m }
            )
            .ToArray();

        var result = new List<PetViewModel>();

        foreach (var info in petInfos) {
            var team = context.Teams
                .SingleOrDefault(t => t.Members.Any(m => m.Id == info.Member.Id));

            result.Add(new PetViewModel {
                Name = info.Pet.Name,
                TeamItIndirectlyBelongsTo = team?.Name,
            });
        }

        return result.ToArray();
    }
}

However, this has a "SELECT N+1" issue in there.

Is there a way to create just one EF query to get the desired result, without changing the entities?

PS. If you prefer a "plug and play" repro containing the above, see this gist .

Answer 1

You've made the things quite harder by not providing the necessary navigation properties, which as @Evk mentioned in the comments do not affect your database structure, but allow EF to supply the necessary joins when you write something like pet.Member.Team.Name (what you need here).

The additional problem with your model is that you don't have a navigation path neither from Team to Pet nor from Pet to Team since the "joining" entity Member has no navigation properties.

Still it's possible to get the information needed with a single query in some not so intuitive way by using the existing navigation properties and unusual join operator like this:

var result = (
    from team in context.Teams
    from member in team.Members
    join pet in context.Pets on member.Id equals pet.Member.Id
    where pet.Name.Contains(pattern)
    select new PetViewModel
    {
        Name = pet.Name,
        TeamItIndirectlyBelongsTo = team.Name
    }).ToArray();