该问题的解决方案是使用DbContext中的AddRange()方法手动将所需的关联实体添加到中间表中，然后将这些更改保存到数据库中。代码示例如下：

[HttpPost]
public async Task CreateBook([FromBody] BookDTO bookDto)
{
    var newBook = new Book
    {
        Title = bookDto.Title,
        Author = bookDto.Author,
        Genres = new List()
    };
 
    foreach (var genreId in bookDto.GenreIds)
    {
        var genre = await _context.Genres.SingleOrDefaultAsync(g => g.Id == genreId);
        newBook.Genres.Add(genre);
    }
 
    _context.Add(newBook);
    await _context.SaveChangesAsync();
 
    return Ok();
}