c# - Entity Framework - 如何删除链接元素

Question

我有以下模型:

我有一个实体空缺对象 (ef_entity)，由 DB 提供，它已经有一个链接的交换元素(也由 DB 提供)。我只想删除它:

        if (ef_vacancy.Swaps != null)
            ef_vacancy.Swaps.Clear();

当我想执行 SaveChanges 时出现错误:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

因此， Entity Framework 尝试将“null”设置为 Swap.VacancyID 表而不是删除记录。但我想删除。如果我尝试执行以下操作:

        var swap_to_delete = (from i in _db.Swaps where i.VacancyID == ID.Value select i).FirstOrDefault();
        if (swap_to_delete != null && _db.Entry(swap_to_delete).State != EntityState.Deleted)
            _db.Swaps.Remove(swap_to_delete);

它有时有效，有时无效!我不明白为什么它有效，为什么不有效。如何正确完成这个简单的 Action ？

Answer 1

这应该可以回答您的问题: https://stackoverflow.com/a/2058787/1154763

答案的文本(归功于 Craig Stuntz ):

Clear() 删除对实体的引用，而不是实体本身。

如果您希望这始终是相同的操作，您可以处理 AssociationChanged:

Entity.Children.AssociationChanged += 
new CollectionChangeEventHandler(EntityChildrenChanged);
Entity.Children.Clear();            

private void EntityChildrenChanged(object sender,
    CollectionChangeEventArgs e)
{
    // Check for a related reference being removed. 
    if (e.Action == CollectionChangeAction.Remove)
    {
        Context.DeleteObject(e.Element);
    }
}

您可以使用分部类将其构建到您的实体中。