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();
Many-to-many relationships require a collection navigation property on both sides. They will be discovered by convention like other types of relationships. The way this relationship is implemented in the database is by a join table that contains foreign keys to both Post and Tag .
When you need to establish a many-to-many relationship between two or more tables, the simplest way is to use a Junction Table. A Junction table in a database, also referred to as a Bridge table or Associative Table, bridges the tables together by referencing the primary keys of each data table.
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);
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