How to remove child entities from parent collection on parent update in Entity Framework Core?-CodePudding

I have a following problem: when I try to update Question and provide a new keyword in Keywords property and the keyword isn't yet related with the question it is being properly added. Lets say there are two keywords mapped to the question. Then I send the update request with one keywords that is not related to the question yet. It results in Question being mapped to three keywords. When I send empty list of keywords, nothing changes. What I would like to happen is when I send one keyword - then it is just this one related to the question and rest is removed. How can I achieve the following ? Already I have tried adding "QuestionKeyword" segment inside Configure method for Keyword but it didn't change much. Also what happens is that keywords are being removed from the entity inside C# application, but when I add a dummy line of code with the same question being fetched from context again, then its question loads again after calling Include() - since they were never removed.

Consider the code below:

public class Keyword
{
        public string Name { get; set; }
        public List<Question> Questions { get; set; }
}

public class Question : AuditableEntity
{
        public int Id { get; set; }
        public string Content { get; set; }
        public string Answer { get; set; }
        public int Rating { get; set; } 
        public IList<Keyword> Keywords { get; set; } = new List<Keyword>(); 
}

This entities are configured in the following way:

public void Configure(EntityTypeBuilder<Keyword> builder)
{
        builder.Ignore(entity => entity.DomainEvents);
        builder.HasKey(entity => entity.Name);

        builder.HasMany(entity => entity.Questions)
            .WithMany(entity => entity.Keywords)
            .UsingEntity<Dictionary<string, object>>(
                       "QuestionKeyword",
                       j => j.HasOne<Question>().WithMany().OnDelete(DeleteBehavior.Cascade),
                       j => j.HasOne<Keyword>().WithMany().OnDelete(DeleteBehavior.Cascade));
 } 


public void Configure(EntityTypeBuilder<Question> builder)
{ 
        builder.HasKey(entity => entity.Id);
}

And this is handle method which updates the question. It was simplified so it just removes the question keywords for presentation purposes.

public async Task<Unit> Handle(UpdateQuestionCommand request, CancellationToken cancellationToken)
{
        Question question = await _context.Questions.Include(q => q.Keywords).AsNoTracking().SingleOrDefaultAsync(q => q.Id == request.Id, cancellationToken);

        if (question == null)
            throw new NotFoundException($"Id = {request.Id}");
        
        question.Keywords = null;
        _context.Questions.Update(question);
        
        question.DomainEvents.Add(new QuestionUpdatedEvent(question));

        await _context.SaveChangesAsync(cancellationToken);
        return Unit.Value;
}

CodePudding user response：

You must clear all keywords of the question then add new keywords into your question like this:

Question question = await _context.Questions.Include(q => q.Keywords).SingleOrDefaultAsync(q => q.Id == request.Id, cancellationToken);

question.Keywords.Clear();

....

But notice that AsNoTracking() is removed!

Using AsNoTracking() means that fetched objects changes dont matter for you and EF will ignore changes!