The instance of entity type 'x' cannot be tracked because another instance with the same key value for {'a', 'b'} is already being tracked - entity-framework

Question

I got the error in the title while editing relational table between many-to-many relationship. It will not let duplicate on table, so I try to remove rows and then create new one but it didn't work.
public void Update(ThermoformProduct entity, int[] thermoformCategoryIds)
{
using (var context = new ShopContext())
{
var product = context.ThermoformProducts
.Include(i => i.ThermoformProductCategories)
.FirstOrDefault(i => i.ProductId == entity.ProductId);
if (product != null)
{
product.Code = entity.Code;
product.Culture = entity.Culture;
product.Renk = entity.Renk;
product.UstGenislik = entity.UstGenislik;
product.UstCap = entity.UstCap;
product.AltCap = entity.AltCap;
product.TbCap = entity.TbCap;
product.Yukseklik = entity.Yukseklik;
product.Hacim = entity.Hacim;
product.TamHacim = entity.TamHacim;
product.Baski = entity.Baski;
product.SosisIciAdet = entity.SosisIciAdet;
product.KoliIciAdet = entity.KoliIciAdet;
product.ImageUrl = entity.ImageUrl;
product.ThermoformProductCategories.RemoveAll(s=>s.ProductId == product.ProductId);
product.ThermoformProductCategories = thermoformCategoryIds.Select(catid => new ThermoformProductCategory()
{
ProductId = product.ProductId,
ThermoformProduct = product,
CategoryId = catid,
ThermoformCategory = context.ThermoformCategories.Where(i => i.CategoryId == catid).FirstOrDefault()
}).ToList();
context.SaveChanges();
}
}
}

Answer 1

EF cannot track two different instance of an entity type with same primary key. You have included the related ThermoformProductCategory entities, so they are being tracked by the context. When you remove them, they are cleared from the ThermoformProductCategories property of that product, but they are not removed from the context, and are still being tracked. Finally when you create the new list of ThermoformProductCategory, some of the new ones' primary key are matching the previous ones' (which already exist in the context)
Since you are creating the entire list again, you don't need to fetch the related entities in the first place. Simply assign a new list and EF will replace the entire list of related entities -
var product = context.ThermoformProducts.FirstOrDefault(i => i.ProductId == entity.ProductId);
if (product != null)
{
// set all the properties
product.ThermoformProductCategories = thermoformCategoryIds.Select(catid => new ThermoformProductCategory()
{
ProductId = product.ProductId,
CategoryId = catid
}).ToList();
context.SaveChanges();
}

Answer 2

Two things:
The filter is redundant - the ThermoformProductCategories navigation property should already be filtered.
product.ThermoformProductCategories.RemoveAll(s=>s.ProductId == product.ProductId);
instead do this:
product.ThermoformProductCategories.RemoveAll(); // Or .Clear()
Don't set navigation properties in this case, just the foreign key values - this should resolve your issue:
Instead of:
product.ThermoformProductCategories = thermoformCategoryIds.Select(catid => new ThermoformProductCategory()
{
ProductId = product.ProductId,
ThermoformProduct = product,
CategoryId = catid,
ThermoformCategory = context.ThermoformCategories.Where(i => i.CategoryId == catid).FirstOrDefault()
}).ToList();
Do:
product.ThermoformProductCategories = thermoformCategoryIds.Select(catid => new ThermoformProductCategory()
{
ProductId = product.ProductId, // Even this might be redundant since you're adding to the product navigation property list.
CategoryId = catid
}).ToList();

Answer 3

both answers was solution here is my final form if anyone gets trouble
public void Update(ThermoformProduct entity, int[] thermoformCategoryIds)
{
using (var context = new ShopContext())
{
var product = context.ThermoformProducts
.FirstOrDefault(i => i.ProductId == entity.ProductId);
if (product != null)
{
product.Code = entity.Code;
product.Culture = entity.Culture;
product.Renk = entity.Renk;
product.UstGenislik = entity.UstGenislik;
product.UstCap = entity.UstCap;
product.AltCap = entity.AltCap;
product.TbCap = entity.TbCap;
product.Yukseklik = entity.Yukseklik;
product.Hacim = entity.Hacim;
product.TamHacim = entity.TamHacim;
product.Baski = entity.Baski;
product.SosisIciAdet = entity.SosisIciAdet;
product.KoliIciAdet = entity.KoliIciAdet;
product.ImageUrl = entity.ImageUrl;
var cmd = "delete from ThermoformProductCategory where ProductId=#p0";
context.Database.ExecuteSqlRaw(cmd, product.ProductId);
product.ThermoformProductCategories = thermoformCategoryIds.Select(catid => new ThermoformProductCategory()
{
ProductId = product.ProductId,
CategoryId = catid,
ThermoformCategory = context.ThermoformCategories.Where(i => i.CategoryId == catid).FirstOrDefault()
}).ToList();
context.Entry(product).State = EntityState.Modified;
context.SaveChanges();
}
}
}