Entity Framework Core 添加具有已保存依赖项的实体会引发错误

Question

I have something like this entities:

class Store
{
    public int Id { get; set; }
    public List<StoreItem> Items { get; set; }
    ...
}
class Item
{
    public int Id { get; set; }
    ...
}
class StoreItem 
{
    public int ItemId { get; set; }
    public Item Item { get; set; }
    public int StoreId { get; set; }
    public Store Store { get; set; }
}

When I added to store saved item and then try to add it to DbSet:

Item item = new Item()
{
    ...
};
using (var transaction = _dbContext.Database.BeginTransaction())
{
    await _dbContext.Items.AddAsync(item); // The item is stored in store StoreItem
    await _dbContext.SaveChangesAsync();
}
...
Store store = new Store()
{
    ...
};
store.Items = new List<StoreItem>()
{
    new StoreItem()
    {
        ItemId = item.Id,
        Item = item,
        Store = store,
    }
};
using (var transaction = _dbContext.Database.BeginTransaction())
{
    await _dbContext.Stores.AddAsync(store);
    await _dbContext.SaveChangesAsync();
}

this code throws exception that:

The instance of entity type 'Item' 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. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the conflicting key values.

I cannot get how to add new Store that have dependency on already saved Item ??

Issue is reproducible on PostgresSQL, here is project example https://github.com/redradist/SO-ReproducedExample

Answer 1

DbSet<T>.Add will assume that all objects that have not been tracked yet are new.

If you want to attach your Item as an existing item, you should do that first, or use context.ChangeTracker.TrackGraph() so you can specify how each object should be attached.

Answer 2

Issue was due to that in example https://github.com/redradist/SO-ReproducedExample I used different objects for the same Many-to-Many relationship:

var storesForItem1 = new List<StoreItemEntity>()
{
    new StoreItemEntity()
    {
        ItemId = item1Entity.Id,
        Item = item1Entity,
        Store = storeEntity,
    }
};
var storesForItem2 = new List<StoreItemEntity>()
{
    new StoreItemEntity()
    {
        ItemId = item2Entity.Id,
        Item = item2Entity,
        Store = storeEntity,
    }
};
var itemsForStore = new List<StoreItemEntity>()
{
    new StoreItemEntity()  // <-- Issue, should be used previous created StoreItemEntity for this relationship
    {
        Item = item1Entity,
        StoreId = storeEntity.Id,
        Store = storeEntity,
    },
    new StoreItemEntity()  // <-- Issue, should be used previous created StoreItemEntity for this relationship
    {
        Item = item2Entity,
        StoreId = storeEntity.Id,
        Store = storeEntity,
    }
};

You should use only one entity instance with the same key for particular object

Here is fixed implementation in branch fixed-example :

https://github.com/redradist/SO-ReproducedExample/tree/feature/fixed-example

Answer 3

- 1

Most of the time this exception is related with Service Registration

And most likely - because you didnt provide more information - your repository/service is registered as a singleton lifetime

-> Startup.cs - services.AddSingleton<IRepository, Repository>();

You have to change it to services.AddScoped<IRepository, Repository>();

- 2

This could be happening because of you are sharing your entity accross multiple DbContext objects.

Please try to use one context for your method.

- 3

AsNoTracking() could be helpful, but while you are adding a new entity I don't think that this is the best situation.

Among all of the above, I think the source of the problem is what I explained in the 1st point.