Cannot add a value to a table, error being thrown on a seemingly unrelated table - sql

Question

I am trying to add a value in my checkoutItems table, in the checkoutItems repository I use this method:
public async Task AddItemAsync(unitItem item, int amount, string selectedOptions)
{
var checkoutItem = await _appDbContext.CheckoutItems.SingleOrDefaultAsync(s => s.unitItem.unitItemId == item.unitItemId && s.CheckoutId == _checkout.CheckoutId);
if (checkoutItem == null)
{
checkoutItem = new CheckoutItem
{
CheckoutId = _checkout.CheckoutId,
unitItem = item,
Amount = 1,
selectedOptions = selectedOptions
};
await _appDbContext.CheckoutItems.AddAsync(checkoutItem);
}
else
{
checkoutItem.Amount++;
}
await _appDbContext.SaveChangesAsync();
}
But when this method tries to save the changes I get an error.
SqlException: Cannot insert explicit value for identity column in table 'Categories' when IDENTITY_INSERT is set to OFF
The issue is I am not trying to insert a value in the Categories table. When I check the SQL Server Profiler I see this code is trying to be executed:
exec sp_executesql N'SET NOCOUNT ON;
INSERT INTO [Categories] ([CategoryId], [CategoryName], [Description])
VALUES (#p0, #p1, #p2);
',N'#p0 int,#p1 nvarchar(450),#p2 nvarchar(4000)',#p0=3,#p1=N'Food',#p2=N'Food served by our kitchen'
I have no idea where this is coming from. Everything was working fine until recently. I made no changes to the code and even restored the database to an earlier state to remove the changes (if any) I had made on the database, but no luck. What is causing this and how can I fix it.
The checkoutItems model looks like this:
public class CheckoutItem
{
public int CheckoutItemId { get; set; }
public unitItem unitItem { get; set; }
public int Amount { get; set; }
public string CheckoutId { get; set; }
public string selectedOptions { get; set; }
}
And there are 2 degrees of separation between a checkoutItem and Category where an Item has a category.
Edit
If you comment out the line unitItem = item, in the add AddItemAsync method, seems to go around the issue. So the issue seems to be around adding the unitItem.
I still don't understand why it is adding a category when add a checkout item. The model for the unitItem looks like this:
public class unitItem
{
[Required]
public int unitItemId { get; set; }
[Required]
public string Name { get; set; }
[DisplayFormat(DataFormatString = "{0:n2}")]
public decimal Price { get; set; }
public bool InStock { get; set; }
public DateTime DateModified { get; set; }
public int CategoryId { get; set; }
[Required]
public virtual Category Category { get; set; }
public string ImageUrl { get; set; }
public string Description { get; set; }
public string OptionFormData { get; set; }
}

Answer 1

in your unitItem class add Required attribute to CategoryId not Category
public class unitItem
{
[Required] //here
public int CategoryId { get; set; }
//[Required] //not here
public virtual Category Category { get; set; }
....
then in AddItemAsync method; set NULL category object if CategoryId is bigger than zero
public async Task AddItemAsync(unitItem item, int amount, string selectedOptions)
{
item.Category = null; //be sure item.CategoryId is not zero
.....