I am writing MVC Web API with .NET. When I add data to one of the models that reference each other, data is also added to the database of the other model.
City:

public class City
 {
     public City()
     {
         Photos = new List<Photo>();
     }
     public int Id { get; set; }
     public int UserId { get; set; }
     public string Name { get; set; }
     public string? Description { get; set; }

     public List<Photo> Photos { get; set; }
     public User? User { get; set; }
 }

User:

public class User
{

    public User()
    {
        Cities = new List<City>();
    }
    public int Id { get; set; }
    public string? UserName { get; set; }
    public byte[]? PasswordHash { get; set; }
    public byte[]? PasswordSalt { get; set; }

    public List<City> Cities { get; set; }
}

Photo:

public class Photo
{
    public int Id { get; set; }
    public int CityId { get; set; }
    public string Url { get; set; }
    public string Description { get; set; }
    public DateTime DateAdded { get; set; }
    public bool IsMain { get; set; }
    public string PublicId { get; set; }

    public City City { get; set; }
}

DataContext : `public class DataContext : DbContext
{
    public DataContext(DbContextOptions<DataContext> options):base(options) 
    {
        
    }

    
    public DbSet<City> Cities { get; set; }
    public DbSet<Photo> Photos { get; set; }
    public DbSet<User> Users { get; set; }
}

And AppRepository :

public class AppRepository : IAppRepository
{
    private DataContext _context;

    public AppRepository(DataContext context)
    {
        _context = context;
    }
    public void Add<T>(T entity) where T : class
    {
        _context.Add(entity);
    }
  

    public void Delete<T>(T entity) where T : class
    {
        _context.Remove(entity);
    }

    public List<City> GetCities()
    {
        var cities = _context.Cities.Include(c=>c.Photos).ToList();
        return cities;
    }

    public City GetCityById(int cityId)
    {
        var city = _context.Cities.Include(c=>c.Photos).FirstOrDefault(c=>c.Id == cityId);
        return city;
    }

    public Photo GetPhoto(int id)
    {
        var photo = _context.Photos.FirstOrDefault(p=>p.Id == id);
        return photo;
    }

    public List<Photo> GetPhotosByCity(int cityId)
    {
        var photos = _context.Photos.Where(p => p.CityId == cityId).ToList();
        return photos;
    }

    public bool SaveAll()
    {
        return _context.SaveChanges() > 0;
    }
}

My CitiesController:

[Route("api/[controller]")]
 [ApiController]
 public class CitiesController : ControllerBase
 {
     private IAppRepository _appRepository;
     private IMapper _mapper;

     public CitiesController(IAppRepository appRepository,IMapper mapper)
     {
         _appRepository = appRepository;
         _mapper = mapper;
     }
     [HttpGet]
     public ActionResult GetCities()
     {
         var cities = _appRepository.GetCities();
         var citiesToReturn = _mapper.Map<List<CityForListDto>>(cities);
         return Ok(citiesToReturn);
     }

     [HttpPost]
     [Route("add")]
     public ActionResult Add([FromBody]City city)
     {
         _appRepository.Add(city);
         _appRepository.SaveAll();
         return Ok(city);
     }

Whenever I post to api/add and add a city, the user information I need to send is also saved user table in the database.Why this is happens ??

I used postman when adding the city and send data in body json format. I thought maybe it wouldn’t be added if I didn’t send the user information, but I got the error that the fields were required. I kept getting this error even though these fields are ALLOW NULLS in the database.Am I misunderstanding the relationships between modules?