How can update a junction table in many to many relation - asp.net

Question

I am going to update schooltypeid which is in the intermediates table, here is my repository class.
public async Task UpdateSchoolsAsync(SchoolUpdateVm schoolUpdateVm)
{
if (_GpsContext != null)
{
var schoolsObj = _GpsContext.School.FirstOrDefault(x => x.ID == Guid.Parse(schoolUpdateVm.id));
var schoolTypeObj = _GpsContext.SchoolsSchoolTypes.FirstOrDefault(x => x.SchoolTypeId == Guid.Parse(schoolUpdateVm.schoolTypeId));
Schools schools = new Schools();
{
schoolsObj.Name = schoolUpdateVm.name;
schoolsObj.Email = schoolUpdateVm.email;
schoolsObj.Phone = schoolUpdateVm.phone;
schoolsObj.Description = schoolUpdateVm.description;
schoolsObj.StateID = Guid.Parse(schoolUpdateVm.stateID);
schoolsObj.CountryId = Guid.Parse(schoolUpdateVm.countryId);
schoolTypeObj.SchoolTypeId = Guid.Parse(schoolUpdateVm.schoolTypeId); //here i can`t update schoolYype
}
_GpsContext.School.Update(schoolsObj);
await _GpsContext.SaveChangesAsync();
}
}
This is my School table in entity framework:
public partial class Schools
{
public Guid ID { get; set; }
public string Name { get; set; }
// Navigations
public ICollection<SchoolsSchoolType> SchoolsSchoolTypes { get; set; }// this is my junction table
}
This is my SchoolsSchoolTypes table:(This is intermediates table)

Answer 1

I think your model in the question is not complete. It lacks the "joining" entity.
That said, if you have a "pure" joining entity, with no additional attributes besides the key (made up of foreign keys), you should add the type to the "SchoolsSchoolTypes" collection in the "Schools" class. The code to add the entity should be something like this:
var schoolsObj = _GpsContext.School.Include(s => s.SchoolsSchoolTypes ).FirstOrDefault(x => x.ID == Guid.Parse(schoolUpdateVm.id)); //Include types to verify isn't already added
if (schoolsObj == null) throw new Exception("School not found");
if(schoolsObj.SchoolsSchoolTypes.Any(st=>st.SchoolTypeId == schoolUpdateVm.schoolTypeId) throw new Exception("School already has this type");
var schoolTypeObj = _GpsContext.SchoolsSchoolTypes.FirstOrDefault(x => x.SchoolTypeId == Guid.Parse(schoolUpdateVm.schoolTypeId));
if (schoolsObj == null) throw new Exception("School type not found");
schoolsObj.SchoolsSchoolTypes.Add(schoolTypeObj);
await _GpsContext.SaveChangesAsync();
If the "joining entity" has additional attributes (I suspect this is the case), then you have to create the new joining entity before adding it to the collection:
var schoolsObj = _GpsContext.School.Include(s => s.SchoolsSchoolTypes ).FirstOrDefault(x => x.ID == Guid.Parse(schoolUpdateVm.id)); //Include types to verify isn't already added
if (schoolsObj == null) throw new Exception("School not found");
if(schoolsObj.SchoolsSchoolTypes.Any(st=>st.SchoolTypeId == schoolUpdateVm.schoolTypeId) throw new Exception("School already has this type");
var schoolTypeObj = _GpsContext.SchoolTypes.FirstOrDefault(x => x.SchoolTypeId == Guid.Parse(schoolUpdateVm.schoolTypeId));
if (schoolsObj == null) throw new Exception("School type not found");
var newSchollType = new SchoolsSchoolTypes()
{
SchollId = Guid.Parse(schoolUpdateVm.id),
SchoolTypeId = Guid.Parse(schoolUpdateVm.schoolTypeId),
OtherProperty = "OtherPropertyValue"
}
schoolsObj.SchoolsSchoolTypes.Add(newSchollType);
await _GpsContext.SaveChangesAsync();