优化部门类属性和方法以支持空值

- 将 `Parent` 和 `ParentName` 属性的返回类型更改为可空类型。
- 添加 `ChildList` 属性以获取子部门列表。
- 新增 `subset` 属性表示是否存在子集。
- 实现 `FindAllByParentId` 方法以根据父级 ID 查找部门，并添加实体缓存逻辑。

XCode/Membership/部门.Biz.cs

@@ -112,11 +112,11 @@ public partial class Department : Entity<Department>, ITenantSource
 
     /// <summary>父级</summary>
     [XmlIgnore, ScriptIgnore, IgnoreDataMember]
-    public Department Parent => Extends.Get(nameof(Department), k => FindByID(ParentID));
+    public Department? Parent => Extends.Get(nameof(Department), k => FindByID(ParentID));
 
     /// <summary>父级</summary>
     [Map(__.ParentID, typeof(Department), __.ID)]
-    public String ParentName => Parent?.ToString();
+    public String? ParentName => Parent?.ToString();
 
     /// <summary>父级路径</summary>
     public String ParentPath
@@ -151,6 +151,18 @@ public partial class Department : Entity<Department>, ITenantSource
             return p + "/" + Name;
         }
     }
+
+    /// <summary>
+    /// 获取子集合
+    /// </summary>
+    [XmlIgnore, ScriptIgnore, IgnoreDataMember]
+    public IEnumerable<Department>? ChildList => Extends.Get(nameof(ChildList), k => FindAllByParentId(ID).OrderBy(e => e.ID));
+
+    /// <summary>
+    ///是否存在子集
+    /// </summary>
+    [XmlIgnore, ScriptIgnore, IgnoreDataMember]
+    public Boolean subset { get; set; }
     #endregion
 
     #region 扩展查询
@@ -231,6 +243,17 @@ public partial class Department : Entity<Department>, ITenantSource
 
         return FindAll(_.TenantId == tenantId);
     }
+
+    /// <summary>根据所属父级Id查找</summary>
+    /// <param name="parentID">所属父级Id</param>
+    /// <returns>实体列表</returns>
+    public static IList<Department> FindAllByParentId(Int32 parentID)
+    {
+        // 实体缓存
+        if (Meta.Session.Count < 1000) return Meta.Cache.FindAll(e => e.ParentID == parentID);
+
+        return FindAll(_.ParentID == parentID);
+    }
     #endregion
 
     #region 高级查询