Entity Framework Core will not update ICollection

Question

Trying to perform an update on an entity with nested list. I keep getting this error no matter what I do. I've tried this: Updating Nested Objects in Entity Framework and this: entity framework update nested objects

Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.

"TypeName": "Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException"

Inserting a new row works just fine. But not update.

public class BuyGroup
{
    [Key]
    public Guid Id { get; set; }

    [Required]
    [Column(TypeName = "nvarchar(150)")]
    public string Name { get; set; }

    public virtual ICollection<Item> Items { get; set; } = new List<Item>();
}

public class Item
{
    [Key]
    public Guid Id { get; set; }

    [Required]
    [Column(TypeName = "nvarchar(100)")]
    public string Name { get; set; }

    public BuyGroup BuyGroup { get; set; }
}

Repository:

public async Task<Guid> Save(Entities.BuyGroup model)
{
    using (var dc = _dataContext())
    {
        // this is ok, i get existing item with Items collection populated
        var existing = await Get(model.Id); 

        if (existing != null)
        {
            existing.Name = model.Name;
            ... // overwrite properties

            existing.Items = model.Items; // overwrite Items colletion

            dc.BuyGroups.Update(existing).State = EntityState.Modified;
        } 
        else 
        {
            await dc.BuyGroups.AddAsync(model);
        }

        // blows up here when existing != null
        await dc.SaveChangesAsync();
    }
}

EDIT:

Adding Get() method

{
    using (var dc = _dataContext())
    {
        return await dc.BuyGroups.FirstOrDefaultAsync(x => x.Id == id);
    }
}

EDIT2:

Using the same context still does not solve my issue:

using (var dc = _dataContext())
{
    var existing = await dc.BuyGroups.FirstOrDefaultAsync(x => x.Id == id); // same context
    if (existing != null)
    {

        existing.Items.Add(new Item{ .....}):
        dc.ByGroups.Entry(existing).State = EntityState.Modified;


    } else {
        await dc.BuyGroups.AddAsync(model);
    }
    await dc.SaveChangesAsync();
}

Answer 1

You want to assign All the Items ins your model, to just one Entity in another table? This is a design error.

Also I'm not sure you can replace an entire collection in one entity. You have to clear them and add new items. You are assigning a DbSet to existing. Items, that's definitely impossible.

Edit Another thing: You create a new Context for 'existing' and you attach something from an other source. Maybe your Items come from another context. They need to be attached to this dc context.

Answer 2

I'm under the impression that since you're invoking a function _dataContext(), instead of calling to a protected instance of the just _dataContext, You're probably creating a new context in the get and save methods.

Since, get and save methods are using seperate DbContexts so when you existing.Items = model.Items; You're using items that are attached to a seperate context.

There are a lot of ways to solve this, but personally I would just create protected methods that accept the dbContext, so you don't have to worry about attaching the entities to the context.

protected BuyGroup GetImplementation(MyDbContext context, int id)
{
    return await context.BuyGroups.FirstOrDefaultAsync(x => x.Id == id);
}

Then in your save method you can just call instead:

var existing = await this.GetImplementation(dc, model.Id);

Edit For Edit

You're setting the new item to modified instead of added

existing.Items.Add(new Item{ .....}):
//You shouldn't do this for added items
//dc.ByGroups.Entry(existing).State = EntityState.Modified;

Answer 3

This is a long shot, I'm not liable if your PC explodes, try this:

public async Task<Guid> Save(Entities.BuyGroup model)
{
    using (var dc = _dataContext())
    {
        var existing = await Get(model.Id); 
        if (existing != null)
        {
            dc.ByGroups.Entry(model).State = EntityState.Modified;

        } else {
            await dc.BuyGroups.AddAsync(model);
        }
        await dc.SaveChangesAsync();
    }
}