Question

I want to add a relationship between multiple existing entities and another existing entity. Here is my model:

public class Term
{
    public int TermId { get; set; }
    public virtual ICollection<SubForm> SubForms { get; set; }
}

public class SubForm
{
    public int SubFormId { get; set; }
    public virtual ICollection<Term> Terms { get; set; }
}

I have an update repository method as follows:

public IQueryable<Term> GetTerms()
{
    IQueryable<Term> query = db.Terms.AsNoTracking();
    return query;
}

public Term UpdateTerm(Term term, IEnumerable<Expression<Func<Term, object>>> properties)
{
    if (term.TermId == 0)
    {
        throw new InvalidOperationException("Term does not exist");
    }
    db.Terms.Attach(term);
    if (properties != null)
    {
        foreach (var selector in properties)
        {
            string propertyName = Helpers.PropertyToString(selector.Body);
            db.Entry(term).Property(propertyName).IsModified = true;
        }
    }
    db.SaveChanges();
    return term;
}

Now I assume this would work when I make this call in my service layer:

public void AddFormToTerm(int termId, int formId)
{
    var term = termsRepository.GetTerms().FirstOrDefault(t => t.TermId == termId);
    var subForms = termsRepository.GetSubForms().Where(t => t.FormId == formId);

    //I assume this would work by adding existing forms to an existing term.
    foreach (var subForm in subForms)
    {
        term.SubForms.Add(subForm);
    }

    termsRepository.UpdateTerm(term, null);
}

Unfortunately, this doesn’t get updated, there is nothing in the intermediate table when I checked the database. No exception was also thrown.

Answer 1

Using AsNoTracking in this case is the problem. Without AsNoTracking it will work. You must keep in mind that you can update a many-to-many relationship only with the change tracking mechanism. But in your code the EF context will know about term and the SubForms collection for the first time when you call Attach in your UpdateTerm method. EF does not notice that you did add the SubForms to the term because those entities were not attached to the context (since you used AsNoTracking = “EF, please do not attach to the context!”). But after Attach nothing happened anymore before you called SaveChanges = No change = No database commands.
So removing AsNoTracking (or creating another method or a parameter to load with tracking) is the best option. Everything else will involve ugly “tricks” like this:

public Term UpdateTerm(Term term, ...)
{
    //...

    // Restore the state before adding the subforms = current state in DB
    var tempSubForms = term.SubForms;
    term.SubForms = null;

    // Inform EF about this state = term exists, subforms exist
    // in DB but no relationships
    db.Terms.Attach(term);
    foreach (var subForm in tempSubForms)
        db.SubForms.Attach(subForm);

    // Change the state: EF change tracking recognizes this
    term.SubForms = tempSubForms;

    //...

    // EF now will send INSERT statements for the join table
    db.SaveChanges();
    return term;
}