Asp.net core 2.0 The instance of entity type 'X' cannot be tracked because another instance with the key value 'Id:x' is already being tracked - entity-framework

Question

I have been working on a shop site project, using asp.net core spa templates provided with the latest VS2017, and have come across an issue that I haven't had before, possibly because until now my apps were quite simple!
I know what the problem is and where, I just can't fix it. I have a product model which has a collection of "Attributes" and a collection of "Variations" (different colour size, etc) and those variations also have attributes, so if the same Attribute shows up in the Variation (VAttributes), as is already in the main "Attributes" I get the error
InvalidOperationException: The instance of entity type
'ProductAttribute' cannot be tracked because another instance with the
key value 'Id:2' is already being tracked. When attaching existing
entities, ensure that only one entity instance with a given key value
is attached.
The best answer I found was here : https://stackoverflow.com/a/19695833/6749293
Unfortunately, even with the above check I got the error, I even tried making a list of attached attributes, and if the vattribute matched one of the items in the list, I didn't attach it. In fact I found that even if I don't attach (_context.attach()) any of the vAttributes, it still throws the error!.
Here's the code in question:
public async Task<Product> Create(Product product)
{
try
{
foreach (var variation in product.Variations)
{
foreach (var vAttr in variation.VAttributes)
{
bool isDetached = _context.Entry(vAttr).State == EntityState.Detached;
if (isDetached)
_context.Attach(vAttr);
}
}
foreach (var attribute in product.Attributes)
{
bool isDetached = _context.Entry(attribute).State == EntityState.Detached;
if (isDetached)
_context.Attach(attribute);
}
foreach (var category in product.Categories)
{
_context.Attach(category);
_context.Attach(category).Collection(x => x.Children);
}
_context.Products.Add(product);
await Save();
return product;
}
catch (Exception)
{
throw;
}
}
The models for the 3 objects are as follows:
public class Product
{
[Key, DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
public string Name { get; set; }
public decimal Price { get; set; }
public string Description { get; set; }
public string StockRef { get; set; }
public DateTime? LastModified { get; set; }
//image needed
public ICollection<ProductCategory> Categories { get; set; }
public ICollection<ProductAttribute> Attributes { get; set; }
public ICollection<ProductVariation> Variations { get; set; }
public Product()
{
Attributes = new List<ProductAttribute>();
Variations = new List<ProductVariation>();
Categories = new List<ProductCategory>();
}
}
Variation:
public class ProductVariation
{
[Key, DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
public DateTime? LastModified { get; set; }
public virtual ICollection<ProductAttribute> VAttributes { get; set; }
//needs images
public decimal VPrice { get; set; }
public string VStockRef { get; set; }
}
Finally the Attribute:
public class ProductAttribute
{
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
public string Name { get; set; }
[ForeignKey("AttributeCategory")]
public int AttributeCategoryId { get; set; }
public virtual AttributeCategory AttributeCategory { get; set; }
}
Most help I found when searching was more related to having repo's injected as singletons, or HttpPut methods where the code had check for existence omitting the .AsNoTracking() or it was a mistake that they had the second instance in some way, where I am aware of the second instance, I just don't know how to prevent it from being tracked!
EDIT: I found that adding a foreign key on the ProductVariation model to the Product that was being created failed as it was only a temp key!? anyway removed it from the variation model, so have updated my code. Also thought I'd add one of my earler failed attempts, that led to all of the foreach loops.
_context.AttachRange(product.Attributes);
_context.AttachRange(product.Categories);
_context.AttachRange(product.Variations);
_context.Add(product);

Answer 1

I believe you can allow EF to handle the tracking.
public virtual bool Create(T item)
{
try
{
_context.Add(item);
_context.SaveChanges();
return true;
}
catch (Exception e)
{
return false;
}
}
This allows for you to save the entire object structure without worring about attaching items.
var newProduct = new Product();
newProduct.Categories.Add(cat);
newProduct.Attributes.Add(att);
newProduct.Variations.Add(vari);
Create(newProduct);