Entity Framework Core navigations not updating - entity-framework-core

Question

I'm using Entity Framework Core v5.0.3 for this.
I'm making a simple information app for users where each user can have favourite colours.
Person model
public class Person
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int PersonId { get; set; }
[MaxLength(50)]
public string FirstName { get; set; }
[MaxLength(50)]
public string LastName { get; set; }
public bool IsAuthorised { get; set; }
public bool IsValid { get; set; }
public bool IsEnabled { get; set; }
public virtual ICollection<FavouriteColour> FavouriteColours { get; set; }
}
Favourite colour model
public class FavouriteColour
{
[Key]
public int PersonId { get; set; }
public virtual Person Person { get; set; }
[Key]
public int ColourId { get; set; }
public virtual Colour Colour { get; set; }
}
Colour model
public class Colour
{
[Key]
public int ColourId { get; set; }
public string Name { get; set; }
public bool IsEnabled { get; set; }
public virtual IList<FavouriteColour> FavouriteColours { get; set; }
}
In my database context I've defined them as so
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<FavouriteColour>()
.HasKey(c => new { c.PersonId, c.ColourId });
modelBuilder.Entity<FavouriteColour>()
.HasOne(fc => fc.Person)
.WithMany(p => p.FavouriteColours)
.HasForeignKey(p => p.PersonId);
modelBuilder.Entity<FavouriteColour>()
.HasOne(fc => fc.Colour)
.WithMany(c => c.FavouriteColours)
.HasForeignKey(c => c.ColourId);
}
Now in the app a user can add or remove favourite colours so a new user object is received in the controller which calls the repository update
public async Task<IActionResult> PutPerson(int id, Person person)
{
peopleRepository.Update(person);
}
The repository then does the update
public void Update(Person person)
{
_context.Update(person);
}
After doing await _context.SaveChangesAsync(); none of the colours change. I thought the purpose of all these models and such was so the colours would change automatically?
I setup a FavouriteColourRepository instead to do the updates like this
public async Task<bool> Update(int personId, ICollection<FavouriteColour> favouriteColours)
{
// empty their favourite colours
_context.FavouriteColours.RemoveRange(_context.FavouriteColours.Where(fc => fc.PersonId == personId);
// add new favourite colours
_context.FavouriteColours.AddRange(favouriteColours);
return true;
}
And I changed my controller to this
public async Task<IActionResult> PutPerson(int id, Person person)
{
peopleRepository.Update(person);
bool valid = await favouriteColourRepository.Update(id, person.FavouriteColours);
}
But for some reason I can't figure out
_context.FavouriteColours.RemoveRange(_context.FavouriteColours.Where(fc => fc.PersonId == personId);
actually alters the favouriteColours parameter and forces the last state into it which creates duplicate primary keys and the inserting fails.
So why do the new favourite colours never get inserted and why is my favouriteColours parameter being edited when I'm trying to clear all the colours a user already has?

Answer 1

why do the new favourite colours never get inserted
The call _context.Update(person); puts the Person entity and all is related FavouriteColour entity in Modified state. Hence, on the next SaveChanges call EF is supposed to submit update commands to the database.
If you modify any property of the Person entity, you'll find that the person has been updated correctly. The issue with the FavouriteColour entity is that it contains nothing but the primary key properties, and EF does not modify the key property (or any property that is part of a composite primary key). You can test this with the following code -
var fc = dbCtx.FavouriteColours
.FirstOrDefault(p => p.PersonId == 1 && p.ColourId == 4);
fc.ColourId = 6; // CoulourId is part of primary key
dbCtx.SaveChanges();
and you will be met with an exception -
The property 'FavouriteColour.ColourId' is part of a key and so cannot
be modified or marked as modified. ...
Therefore, EF will not even generate any update command for the FavouriteColours, even though all of them are marked as Modified.
To update the Person and all its FavouriteColour you need to do something like -
// fetch the exising person with all its FavouriteColours
var existingPerson = dbCtx.Persons
.Include(p => p.FavouriteColours)
.FirstOrDefault(p => p.Id == id);
// modify any properties of Person if you need
// replace the existing FavouriteColours list
existingPerson.FavouriteColours = person.FavouriteColours;
// save the changes
dbCtx.SaveChanges();
This will -
delete any FavouriteColour that are in the existing list but not in the new list
insert any FavouriteColour that are in the new list but not in the existing list.
With your repositories in place, its up to you how you implement this.
why is my favouriteColours parameter being edited when I'm trying to
clear all the colours a user already has
Before this operation you called _context.Update(person);. Therefore, Person is now being tracked in Modified state as an existing entity. Then when you called -
_context.FavouriteColours.RemoveRange(_context.FavouriteColours.Where(fc => fc.PersonId == personId));
the _context.FavouriteColours.Where(fc => fc.PersonId == personId) part, fetched the existing FavouriteColours of that Person from the database. So these fetched FavouriteColours are added to the person's FavouriteColours list (because it is being tracked).
For example, lets say in your controller you received a Person entity with 4 FavouriteColour, and the database has 3 FavouriteColour for this person. After calling _context.Update(person);, the person is being tracked with a list of 4 FavouriteColour. Then when you fetched this person's existing FavouriteColours from the database, the person is being tracked with a list of total 7 FavouriteColour.
I hope that shades any light.