Entity framework core update many to many - entity-framework

Question

We are porting our existing MVC6 EF6 application to Core.
Is there a simple method in EF Core to update a many-to-many relationship?
My old code from EF6 where we clear the list and overwrite it with the new data no longer works.
var model = await _db.Products.FindAsync(vm.Product.ProductId);
model.Colors.Clear();
model.Colors = _db.Colors.Where(x =>
vm.ColorsSelected.Contains(x.ColorId)).ToList();

Answer 1

This will work for you.
Make a class to have the relationship in:
public class ColorProduct
{
public int ProductId { get; set; }
public int ColorId { get; set; }
public Color Color { get; set; }
public Product Product { get; set; }
}
Add a ColorProduct collection to your Product and Color classes:
public ICollection<ColorProduct> ColorProducts { get; set; }
Then use this extension I made to remove the unselected and add the newly selected to the list:
public static void TryUpdateManyToMany<T, TKey>(this DbContext db, IEnumerable<T> currentItems, IEnumerable<T> newItems, Func<T, TKey> getKey) where T : class
{
db.Set<T>().RemoveRange(currentItems.Except(newItems, getKey));
db.Set<T>().AddRange(newItems.Except(currentItems, getKey));
}
public static IEnumerable<T> Except<T, TKey>(this IEnumerable<T> items, IEnumerable<T> other, Func<T, TKey> getKeyFunc)
{
return items
.GroupJoin(other, getKeyFunc, getKeyFunc, (item, tempItems) => new { item, tempItems })
.SelectMany(t => t.tempItems.DefaultIfEmpty(), (t, temp) => new { t, temp })
.Where(t => ReferenceEquals(null, t.temp) || t.temp.Equals(default(T)))
.Select(t => t.t.item);
}
Using it looks like this:
var model = _db.Products
.Include(x => x.ColorProducts)
.FirstOrDefault(x => x.ProductId == vm.Product.ProductId);
_db.TryUpdateManyToMany(model.ColorProducts, vm.ColorsSelected
.Select(x => new ColorProduct
{
ColorId = x,
ProductId = vm.Product.ProductId
}), x => x.ColorId);

Answer 2

In order to avoid the LINQ hell in the above answer, the templated "Except" method can be rewritten as such:
public static IEnumerable<TEntity> LeftComplementRight<TEntity, TKey>(
this IEnumerable<TEntity> left,
IEnumerable<TEntity> right,
Func<TEntity, TKey> keyRetrievalFunction)
{
var leftSet = left.ToList();
var rightSet = right.ToList();
var leftSetKeys = leftSet.Select(keyRetrievalFunction);
var rightSetKeys = rightSet.Select(keyRetrievalFunction);
var deltaKeys = leftSetKeys.Except(rightSetKeys);
var leftComplementRightSet = leftSet.Where(i => deltaKeys.Contains(keyRetrievalFunction.Invoke(i)));
return leftComplementRightSet;
}
Furthermore the UpdateManyToMany method can be updated to include entities that have been modified as such:
public void UpdateManyToMany<TDependentEntity, TKey>(
IEnumerable<TDependentEntity> dbEntries,
IEnumerable<TDependentEntity> updatedEntries,
Func<TDependentEntity, TKey> keyRetrievalFunction)
where TDependentEntity : class
{
var oldItems = dbEntries.ToList();
var newItems = updatedEntries.ToList();
var toBeRemoved = oldItems.LeftComplementRight(newItems, keyRetrievalFunction);
var toBeAdded = newItems.LeftComplementRight(oldItems, keyRetrievalFunction);
var toBeUpdated = oldItems.Intersect(newItems, keyRetrievalFunction);
this.Context.Set<TDependentEntity>().RemoveRange(toBeRemoved);
this.Context.Set<TDependentEntity>().AddRange(toBeAdded);
foreach (var entity in toBeUpdated)
{
var changed = newItems.Single(i => keyRetrievalFunction.Invoke(i).Equals(keyRetrievalFunction.Invoke(entity)));
this.Context.Entry(entity).CurrentValues.SetValues(changed);
}
}
which uses another custom templated "Intersect" function to find the intersection of the two sets:
public static IEnumerable<TEntity> Intersect<TEntity, TKey>(
this IEnumerable<TEntity> left,
IEnumerable<TEntity> right,
Func<TEntity, TKey> keyRetrievalFunction)
{
var leftSet = left.ToList();
var rightSet = right.ToList();
var leftSetKeys = leftSet.Select(keyRetrievalFunction);
var rightSetKeys = rightSet.Select(keyRetrievalFunction);
var intersectKeys = leftSetKeys.Intersect(rightSetKeys);
var intersectionEntities = leftSet.Where(i => intersectKeys.Contains(keyRetrievalFunction.Invoke(i)));
return intersectionEntities;
}