How do I add/update the child of a child in C# using Entity Framework Core?

Question

This answer was useful in updating child entities when a parent is updated. I'm now trying to also update a child of a child. The below works for finding what child records, if any, belong to a parent so records can be added/updated.

var calFile = await innerContext.Calibrate
    .Where(x => x.DataId == dataFile.Id)
    .Include(x => x.Symptom)                              
    .SingleOrDefaultAsync();

This does not work for adding/updating the child of a child:

var calFile = await innerContext.Calibrate
    .Where(x => x.DataId == dataFile.Id)
    .Include(x => x.Symptom) 
    .Include(x => x.SymptomQuestions                             
    .SingleOrDefaultAsync();

My entities are as follows:

public class Calibrate : IAnalyticsSection
{
    public int Id { get; set; }
    public Guid DataFileId { get; set; }
    public bool TestType { get; set; }
    public decimal Height { get; set; }
    public decimal CalibratedHeadPosition { get; set; }
    public decimal LeftHandPositionTPose { get; set; }
    public decimal RightHandPositionTPose { get; set; }
    public decimal LeftHandSpherePos { get; set; }
    public decimal RightHandSpherePos { get; set; }

    public ICollection<Symptom> Symptoms { get; set; } = new List<Symptom>();
    public virtual DataFile DataFile { get; set; }
}

public class Symptom
{
    public int Id { get; set; }
    public int CalibeId { get; set; }
    public int SymptomSeverity { get; set; }



    public virtual ICollection<SymptomQuestions> SymptomQuestions { get; set; } = new List<SymptomQuestions>();

    public virtual Calibrate Calibrate { get; set; }
}

public class SymptomQuestions
{
    public int Id { get; set; }
    public int SymptomsId { get; set; }
    public int Question { get; set; }
    public int Answer { get; set; }

    public virtual Symptom Symptoms { get; set; }

}

Calibrate can have several Symptoms, each of which will have 5 questions.

How can this be done?

Answer 1

Maybe it's as simple as using .ThenInclude(..) :

var calFile = await innerContext.Calibrate
    .Where(x => x.DataId == dataFile.Id)
    .Include(x => x.Symptom) 
        .ThenInclude(x => x.SymptomQuestions)
    .SingleOrDefaultAsync();

But maybe we need to see the definition of the relations (FluentAPI?), too?

Or your issue is related to a recent breaking change of EF Core 5.0 ( Non-null reference navigations are not overwritten by queries ): .Include(...) will only set related items from the database, if your navigation property is still null. Since you initialize Symptoms and SymptomsQuestions here, .Include() might just do nothing.

Side note: Why call one entity "Symptom" (singular) and the other "SymptomQuestion s " (= plural)? Both seem to represent just one instance.