Including only Id of related entity in entity framework core

Question

I am currently developing an API with ASP.NET Core and Entity framework core with npgsql as database provider. I have two Entities and they have a one to many relation. The thing is that I only want to include the Id's of the child entity in the JSON result that the "Parent Controller" returns.

These are my entities:

public class Meal {
    public int Id { get; set; }

    public string Title { get; set; }
    public string Description { get; set; }

    public string UserId { get; set; }
    public User User { get; set; }

    public List<Picture> Pictures { get; set; }

    public Meal () {
        this.Pictures = new List<Pictures>();
    }
}

public class Picture {
    public int Id { get; set; }

    public int MealId { get; set; }
    public Meal Meal { get; set; }

    public byte[] full { get; set; }
    public byte[] small { get; set; }
}

I am however not sure on how to achieve this. Yesterday I came across another SO question which suggested something like this:

public IActionResult Meals () {
    var meal = this.context.Meals
        .Include(m => m.Pictures.Select(p => p.Id))
        .First();

    return new JsonResult(meal);
}

This however throws an InvalidOperationException. My DbContext is very basic, no onModelConfiguring because this code is following convention for as far as I know and it just has two DbSets of the corresponding types. The foreign keys are also correct in the database and callling something like:

var pictures = dbContext.Pictures.Where(p => p.MealId == mealId).ToList();

Works as expected. I have only included the code which I thought was relevant. If more is needed I will include it, but I think this is completely my limited understanding of the queries.

Thank you for your time!

Answer 1

You don't need to change your DB structure, one option is like so:

        var db = this.context;
        var result = (from meal in db.Meals
                      where meal.<whatever> == "123"
                      select new
                      {
                         Id = meal.Id,
                         Title = meal.Title,
                         Description = meal.Description,
                         //other required meal properties here.
                         PictureIds = meal.Pictures.Select(x => x.Id)
                      }).ToList();

You can do the same thing via lambda as well by using the "Select" method, Linq in such things seem more intuitive to me, however, to each his own... that's your choice.