Many to many relationship - How to delete/modify nested collection items?

[scarybook]

I created many to many relationship and had a problem.
When I created new entity with collection of items it worked fine - items were added, but when I tried to update this entity with new set of items I was getting an error (only when any item of collection was already in DB) .

Sample model classes:

    public class Product : AuditableEntity<int>
    {
        public string Name { get; set; }
        public string Barcode { get; set; }

        [Column(TypeName = "text")]
        public string ImageDataURL { get; set; }

        public string Description { get; set; }
        public decimal Rate { get; set; }
        public int BrandId { get; set; }
        public virtual Brand Brand { get; set; }

        public IEnumerable<Category> Categories { get; set; }
    }

    public class Category : AuditableEntity<int>
    {
        public string Name { get; set; }
        public IEnumerable<Product> Products { get; set; }
    } 

AddEditProductCommand class

            if (command.Id == 0)
            {
                var product = _mapper.Map<Product>(command);
                product.Categories = categories;
                if (uploadRequest != null)
                {
                    product.ImageDataURL = _uploadService.UploadAsync(uploadRequest);
                }
                await _unitOfWork.Repository<Product>().AddAsync(product);
                await _unitOfWork.Commit(cancellationToken);
                return await Result<int>.SuccessAsync(product.Id, _localizer["Product Saved"]);
            }
            else
            {
                var product = await _unitOfWork.Repository<Product>().GetByIdAsync(command.Id);
                if (product != null)
                {
                    product.Categories = categories;
                    product.Name = command.Name ?? product.Name;
                    product.Description = command.Description ?? product.Description;
                    if (uploadRequest != null)
                    {
                        product.ImageDataURL = _uploadService.UploadAsync(uploadRequest);
                    }
                    product.Rate = (command.Rate == 0) ? product.Rate : command.Rate;
                    product.BrandId = (command.BrandId == 0) ? product.BrandId : command.BrandId;
                    await _unitOfWork.Repository<Product>().UpdateAsync(product);
                    await _unitOfWork.Commit(cancellationToken);
                    return await Result<int>.SuccessAsync(product.Id, _localizer["Product Updated"]);
                }
                else
                {
                    return await Result<int>.FailAsync(_localizer["Product Not Found!"]);
                }
            }

I needed to change getting an object from DB as below:

var product = await _unitOfWork.Repository<Product>().Entities.Include(x => x.Categories).FirstOrDefaultAsync(x => x.Id == command.Id);