为什么 EF Core 不会在我的数据库中更新此项目？

Question

I'm having trouble updating an item on my database. I've tried several different ways, but nothing seems to work. Here is my latest attempt:

public async Task<Finding>  InsertOrUpdateItemAsync(Item localItem)
{
    using (var context = new AppDbContext())
    {
       context.Items.Update(localItem);
       context.SaveChanges();
    }
}

When SaveChanges executes, I get the following error message: "The instance of entity type ItemStatus cannot be tracked because another instance with the key value ['ItemStatusId: 4'] is already being tracked.

Here are the relevant properties of my Item model:

public class Item
{   
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ItemId { get; set; }

    public ItemStatus InitialStatus { get; set; }

    public ItemStatus FinalStatus {get; set; }
}

And here are the relevant properties from the ItemStatus class:

public class ItemStatus
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ItemStatusId { get; set; }

    public string ItemStatusName{ get; set; }
]

Can anyone tell me what I'm doing wrong? I have checked to make sure I don't have an undisposed context somewhere. Also, when I look at the change tracker, I can see that it is tracking an entry from each row on the ItemStatus table. This doesn't seem right. Shouldn't it only track the ItemStatus that has been assigned to localItem rather than all the related entities?

I am calling the method from here:

savedItem = awaitdataService.InsertOrUpdateFindingAsync(ItemToDisplay);

The various properties of ItemToDisplay are bound to dropdown lists in the UI. I have verified that these properties are being assigned correctly before being sent to the InsertOrUpdateFindingAsync method.

I have also tried a this for the InsertOrUpdate method:

public async Task<Finding>  InsertOrUpdateItemAsync(Item localItem)
{
    using (var context = new AppDbContext())
    {
       Item itemFromDb = context.Items.Where(i => i.ItemId == localItem.ItemId).FirstOrDefault();
       itemsFromDb = localItem;
       context.Items.Update(localItem);
       context.SaveChanges();
    }
}

Answer 1

You apparently save a localItem that refers to two ItemStatus objects both having ItemStatusId = 4. The cause of the error is that these ItemStatus objects are two instances instead of one. The Update command tries to attach both of these instances to the context.

There are two ways to fix this:

1. Make sure that if both statuses in Item are identical they both refer to the same ItemStatus object. Depending on where localItem comes from, this may require JSON serializer settings or a modification in the code that constructs the localItem object.

2. (Preferred) Add the primitive foreign properties InitialStatusId and FinalStatusId to Item and only set these properties, not the references. Then the update is a simple update of scalar properties.

Answer 2

Because

savedItem = awaitdataService.InsertOrUpdateFindingAsync(ItemToDisplay);

this variable is itemtodisplay, you have to read from once and you keep this data in this variable, so your context to database, tracked this record when you read.

public async Task<Finding>  InsertOrUpdateItemAsync(Item localItem)
{
    using (var context = new AppDbContext())
    {
       context.Items.Update(localItem);
       context.SaveChanges();
    }
}

Then here when you sent this variable to there, there is already tracked variable, you tried to track it one more time.

There are 2 solutions:

1. Read this record with .AsNoTracking so it will not be tracked

2. When your update method you need to read one more time, like searching with id and keep it in another variable, and assign the parameters which you want to update to sent variable to new read one. then save it so it will not tracked, because there is a new read, actually you read this with tracking parameter because you don't need to add .AsNoTracking here, but it will create a problem because you are trying to change something on same tracked context.

I will add basic code based on my 2. desc.

    public async Task<Finding>  InsertOrUpdateItemAsync(Item localItem)
    {
        using (var context = new AppDbContext())
        {
           var data = context.Items.Where(x => x.Id == localItem.Id).FirstOrDefault();
           data.Name = localItem.Name;
           context.Items.Update(data);
           context.SaveChanges();
        }
    }

I hope I described it well.

But note: using update method is not the best, because in that example I only want to update name property but it will create query that updates all columns. You should do it with out update, else there is 2 option here, if write update, it says hey EF mark all need to update, so it creates a query that to update all columsn. But this data is already tracked, if you change name like example EF Core knows it so you should only write there, SaveChanges(); or second method is if you want to update only one record, but your record is read with .AsNoTracking , you should access the context, get entries and change the field as modified

public void ContextAttach(TEntity entity)
{
    _dataContext.Attach(entity);
    _dataContext.Entry(entity).Property(Name).IsModified = true;
}

https://blog.oneunicorn.com/2020/01/17/dontcallupdate/