实体如何允许外键在父子关系中？

Question

我有实体

/// <summary>
/// The greoup.
/// </summary>
public class Group
{
    #region Public Properties

    /// <summary>
    /// Gets or sets the group id.
    /// </summary>
    [Key]
    public int GroupId { get; set; }

    /// <summary>
    /// Gets or sets the parent group id.
    /// </summary>     
    public int ParentGroupId { get; set; }

    /// <summary>
    /// Gets or sets the active.
    /// </summary>
    public int Active { get; set; }

    /// <summary>
    /// Gets or sets the description.
    /// </summary>
    public string Description { get; set; }

    /// <summary>
    /// Gets or sets the group guid.
    /// </summary>
    public Guid GroupGuid { get; set; }

    /// <summary>
    /// Gets or sets the order weight.
    /// </summary>
    public int OrderWeight { get; set; }

    /// <summary>
    /// Gets or sets the parent group.
    /// </summary>
    [ForeignKey("ParentGroupId")]
    public virtual Group ParentGroup { get; set; }

    /// <summary>
    /// Gets or sets the groups.
    /// </summary>
    public virtual ICollection<Group> Groups { get; set; }

    #endregion
}

如何允许退出外键。因为当我尝试在ParentGroupId = 0时添加Group。我得到异常

  

无法确定相关操作的有效排序。由于外键约束，模型要求或存储生成的值，可能存在依赖关系。

Answer 1

您正在使用此结构在数据库中创建自引用表。所以ParentGroupId必须是可空的。根节点不能有父节点，但是它产生的所有节点都有父节点，所以你需要使ParentGroupId可以为空。