实体框架包含一对多关系中的单个对象

Question

I'm trying to construct a single query to fetch the list of all Tour objects, with each tour's latest version preloaded (latest being most recently created). See the models below. Each Tour can have multiple associated TourVersion objects

class Tour {
    public Guid TourId { get; set; }
    public virtual TourVersion CurrentTourVersion { get; set; }
    // Other properties etc.
}
class TourVersion {
    public Guid TourId { get; set; }
    public DateTime CreatedOn { get; set; }
    // Other properties etc.
}

I'm able to accomplish what I want running another query for each Tour :

var tours = context.Tours.ToList();

foreach (var tour in tours)
{
    tour.CurrentTourVersion = context.TourVersions
        .Where(t => t.TourId == tour.Id)
        // ToUnixTime is a custom extension method that returns a long
        .OrderByDescending(t => t.CreatedOn.ToUnixTime())
        .FirstOrDefault();
}

I'd like to be able to accomplish this in a single query. Any suggestions?

Answer 1

Entity framework automatically loads related entities for you until the context is alive. You get the list of Tours by

var tours = dbContext.Tours.ToList();

for example and you can get the tour version by calling

tours[i].CurrentTourVersion;

Unless dbContext is disposed somewhere before you get tour version in that case you are going to need eager loading (see here ). This way you get and keep the related objects.

var tours = dbContext.Tours.Include(obj=>obj.CurrentTourVersion).ToList();

Answer 2

Ended up solving it with this:

var tours = (from tour in context.Tours
             join v in context.TourVersions on tour.TourId equals v.TourId
             where v.CreatedOn == context.TourVersions.Select(b => b.CreatedOn).Max()
                select new
                {
                    TourId = tour.Id,
                    CurrentTourVersion = v
                }).ToList().Select(tour => new Tour
                {
                    TourId = tour.Id,
                    CurrentTourVersion = tour.CurrentTourVersion
                });

return tours.ToList();

Explanation of why this works:

• Joins each tour with the most recent version
  • Finds the most recent version by selecting version against maximum date
• Creates an anonymous object with the TourId and CurrentTourVersion fields
• Maps that to a list of Tour objects

EF has a restriction to force you to put the results of a join into an anonymous object. See this answer for more info