无法跟踪实体类型“AppUser”的实例，因为已跟踪另一个具有与 {'Id'} 相同键值的实例

Question

Hopefully someone can help a fairly novice EF/ asp.net core dev...

One of my methods is failing on 'context.Add(item)'

The Model:

public class Component : Item
{
    public int Id { get; set; }
    public string ComponentName { get; set; }
    public string ComponentDescription { get; set; }
}

The method that fails ('_user' is AppUser which is set in the constructor):

public async Task<bool> UpdateComponentOwnerAsync(ComponentOwner componentOwner)
{
    var co = new ComponentOwner();
    bool success = false;
    try
    {
        if (componentOwner.Id == 0)
        {
            co.Component = componentOwner.Component;
            co.User = componentOwner.User;
            co.ModifiedDate = DateTime.Now;
            co.UpdatedBy = "Someone";
        }
        else
        {
            co = componentOwner;
        }

        var coNew = (ComponentOwner)await SaveItem(co);

private async Task<Item> SaveItem(Item item)
{
    using var context = _dbContextFactory.CreateDbContext();
    try
    {
        context.Users.Attach(_user);

        if (item.Id == 0)
        {
            context.Add(item);
            await context.SaveChangesAsync();

        }

The error:

System.InvalidOperationException: 'The instance of entity type 'AppUser' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached.

Removing 'context.Users.Attach(_user)' results in the following SQL Exception, SO this does need to stay in it seems:

SqlException: Violation of PRIMARY KEY constraint 'PK_AspNetUsers'. Cannot insert duplicate key in object 'dbo.AspNetUsers'. The duplicate key value is (7344f007-4463-46ab-b435-651fb9e0e29a). Cannot insert explicit value for identity column in table 'Components' when IDENTITY_INSERT is set to OFF. The statement has been terminated.

I've looked at similar posts and they all suggest that it's due to the scope being incorrect or 'AppUser being tracked. A scope isn't required as far as I can tell however (in my program.cs file, I have the following:

builder.Services.AddDbContextFactory<DBContext>(options => options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")));
builder.Services.AddDbContext<DBContext>(options => options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")));`

and I can't see why AppUser would be being tracked elsewhere.

Hopefully someone can tell me where I'm going wrong!

Answer 1

Your trying to add a record that already exists. Context.Add and Context.Update do exactly what they say.

If you don't fully understand the differences between value and reference types I suggest you start reading.

In UpdateComponentOwnerAsync if componentOwner is an existing record then componentOwner.Id is not 0 .

Therefore

co = componentOwner;

assigns the componentOwner reference to co .

You then call SaveItem(co) which attempts to do a

context.Add(item [actually co])

And fails. co points to the same reference as componentOwner passed in to UpdateComponentOwnerAsync . As you obtained componentOwner from the database in the first place, then you have a duplicate id.

You don't show how you obtained componentOwner and what logic sets it's Id to 0 , so I'm not in a position to suggest a meaningful solution.

A couple of pointers:

1. You should differentiate in your code and data pipeline between Updates and Creates/Adds. It makes the logic more straightforward and easier to understand.
2. Check the return value of context.SaveChangesAsync . You should get back 1 ie one record updated/created.