实体框架：与抽象类的多对多关系，是否在不查询数据库的情况下进行修改？

Question

With Entity Framework, if I have the following model:

class Asset {
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<AssetGroup> Groups { get; set; }
}

class AssetGroup {
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Asset> Assets { get; set; }
}

Adding an existing asset to a given group without having to query the database to load the asset in question is relatively straightforward:

using(var context = new MyContext()) {
    AssetGroup assetGroup = context.AssetGroups.Find(groupId);

    // Create a fake Asset to avoid a db query
    Asset temp = context.Assets.Create();
    temp.Id = assetId;
    context.Assets.Attach(temp);

    assetGroup.Assets.Add(temp);
    context.SaveChanges();
}

However, I now have the problem that I have extended the model so that there are multiple Asset types, implemented as an inheritance hierarchy. In the process, the Asset class has been made abstract, since an asset with no specific type makes no sense:

abstract class Asset {
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<AssetGroup> Groups { get; set; }
}

class SpecificAsset1 : Asset {
    public string OneProperty { get; set; }
}

class SpecificAsset2 : Asset {
    public string OtherProperty { get; set; }
}

The problem is that now the Create() call throws an InvalidOperationException because "Instances of abstract classes cannot be created", which of course makes sense.

So, the question is, how can I update the many-to-many relation without having to go fetch the Asset from the database?

Answer 1

You can use the generic Create<T> method of DbSet<T> to create a derived entity object.

var temp1 = context.Assets.Create<SpecificAsset1>();

and continue from here using temp1 as a stub entity the way you already did.