Error Object Id' is unknown when attempting to save changes. Adding Many to Many to the DB

Question

I have objects with many to many relationship.

public class Executor
{
    public long Id { get; set; }

    public string Name { get; set; }

    public List<Competency> Competency { get; set; }

}

public class Competency
{      

    public long Id { get; set; }

    public string CompetencyName { get; set; }

    public List<Executor> Executor { get; set; }
}

I am using EF Core 5 and PostgreSQL DB. I can`t just add new Executor to DB, first I need to find all competencies in the DB because of this problem.

So, my code now is like this:

public async Task<ServiceResponse<ExecutorDto>> AddExecutor(ExecutorDto newExecutor, long userId)
    {
        var serviceResponse = new ServiceResponse<ExecutorDto>();
        try
        {
            var executor = _mapper.Map<Executor>(newExecutor);
            executor.Competency.Clear();
            executor.Competency = _context.Competencies.Where(i => newExecutor.Competency.Contains(i)).ToList();

            _context.Executors.Add(executor);                
            await _context.SaveChangesAsync();

            ...

But on the Save moment I have error.

The value of 'CompetencyExecutor (Dictionary<string, object>).CompetencyId' is unknown when attempting to save changes. This is because the property is also part of a foreign key for which the principal entity in the relationship is not known.

I was trying to resolve this in many ways, but I can`t find the solution.

Answer 1

Well, it was stupid, the problem was because one of the Competency in the List has Id=0. PostreSQL recognises 0 as NULL. Just need to change Id to 1 or another positive number.