Error Performing Update on Entity Framework

Question

I have the following relationship between Agenda and Contact:

• An Agenda may have N Contacts;

• A contact can be in N Agendas;

In other words, I have an N:M relationship between the Agenda and Contact entities;

When creating an Agenda with the contact list the E.F Add action goes perfectly, but when trying to include a new Contact in an already created Agenda it happens that the SaveChangesreturns 1 but in the base there is no change, that is, in the table Schedulecontact is not adding the IDconcerning the Agenda and the new contact.

Code that adds the new Contact:

public BookModel AddNewContact(ContactModel newcontact, Guid bookId)
{
      Contact contact = null;

      var contatoexiste = _contactRepository.Exist(newcontact.contactId);

      if (contatoexiste)
            contact = _contactRepository.Search(x => x.contactId == 
          newcontact.contactId).FirstOrDefault();

      else
      {
          var newcontactdomain = Mapper.Map<Contact>(newcontact);

         contact = _contactRepository.AddOrUpdate(newcontactdomain);
       }

       try
       {
            var book = _bookRepository.GetById(bookId);

            book.Customers.Add(contact);

            _bookRepository.Update(book);

            return Mapper.Map<BookModel>(book);

        }
        catch (Exception ex)
        {

            throw;
        }

  }

Update Code (Repository):

public T Update(T entity)
{
     _Dbset.Attach(entity);

      Context.Entry(entity).State = EntityState.Modified;

      var result = Context.SaveChanges(); //retorna 1 se ok 

      return entity;
}