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建模一对零或一关系(Z基数)

nhibernate-mapping oop nhibernate

Jamie Ide · 技术社区 · 16 年前

例如,假设我有两个类 Widget 和 WidgetTest . 并非所有小部件都经过测试,而且测试是破坏性的,因此每个小部件最多只能有一个WidgetTest。还假设向小部件添加WidgetTest字段是不合适的。

我希望我的公共界面是:

Widget
    WidgetTest { get; set; }

WidgetTest
    Widget { get; }

模型1:小部件具有WidgetTest属性,并且在数据库中,小部件表具有WidgetTest的唯一约束外键。我的DBA认为这将允许WidgetTest记录在没有小部件的情况下存在。

WidgetTable
    WidgetTestId (FK, UQ)

模型2:Widget有一个WidgetTest的私有集合,并通过从由公共WidgetTest属性控制的集合中添加或删除单个对象来强制0,1关系。数据库将其建模为1:m,WidgetTest对小部件具有唯一约束的外键。我认为这意味着采用模型来适应数据库模式(即,我需要做更多的工作)。

WidgetTestTable
    WidgetId (FK, UQ)

哪种型号更好?使用NHibernate哪个更容易实现?还是有第三条路?

编辑以下是我最终得到的结果:

public class Widget
{
    // This is mapped in NH using a access strategy
    private IList<WidgetTest> _widgetTests = new List<WidgetTest>(1);

    public WidgetTest
    {
        get { return _widgetTests.FirstOrDefault(); }
        set
        {
            _widgetTests.Clear();
            if (value != null)
            {
                _widgetTests.Add(value);
            }
         }
     }
}

4 回复 | 直到 16 年前

nw. 16 年前

我的方法是在映射中建模一对多关系,但将“多”约束到单个项。这允许可选的一对一,并且还保证在保存小部件时保存WidgetTest实例。例如:

public class Widget
{
    /// <summary>
    /// This property is ignored by the NHibernate mappings.
    /// </summary>
    public virtual WidgetTest WidgetTest { get; set; }

    /// <summary>
    /// For easier persistence with NHibernate, this property repackages the
    /// WidgetTest property as a list containing a single item. If an
    /// attempt is made to set this property to a list containing more than
    /// one item, an exception will be thrown. But why bother? Just use the
    /// WidgetTest property.
    /// </summary>
    public virtual IList<WidgetTest> WidgetTests
    {
        get
        {
            IList<WidgetTest> widgetTests = new List<WidgetTest>();
            if (this.WidgetTest != null)
            {
                widgetTests.Add(this.WidgetTest);
            }
            return widgetTests;
        }
        set
        {
            if (value != null && value.Count > 1)
            {
                throw new Exception("The WidgetTests collection may not contain more than one item.");
            }
            else if (value != null && value.Count == 1)
            {
                this.WidgetTest = value[0];
            }
            else
            {
                this.WidgetTest = null;
            }
        }
    }
}

Graham 16 年前

当您说“假设将WidgetTest字段添加到Widget是不合适的”时,您的意思是在域对象中还是在数据库中。如果您希望字段位于数据库中的同一个表中,那么将WidgetTest映射为Widget的一个组件如何?使NHibernate映射文件如下所示:

<class name="Widget" table="Widget">
    ...
    <property name="WidgetProperty"/>
    ...
    <component name="WidgetTest" class="WidgetTest">
        <property name="WidgetTestProperty"/>
    </component>
</class>

给出表格结构:

WidgetTable
    WidgetProperty
    WidgetTestProperty

Community Mohan Dere 9 年前

我还有两个想法 here

将表和映射作为组件连接

Community Mohan Dere 9 年前

这个 answer given by nw. A collection with cascade=âall-delete-orphanâ was no longer referenced by the owning entity instance ".

如果您使用 inverse="true" cascade="all-delete-orphan" 在映射文件中。

get 访问器被调用,并且不会对通过 set IList<WidgetTest> 创建对象时最初传入的引用,无法继续级联。

所以为了解决这个问题,我们需要做点什么 IList<Widgetest> 引用,注意不要取消引用。

public class Widget
{
    public Widget()
    {
        _widgetTests = new List<WidgetTest>();
    }

    /// <summary>
    /// This property is ignored by the NHibernate mappings.
    /// </summary>
    public WidgetTest WidgetTest { get; set; }

    /// <summary>
    /// For easier persistence with NHibernate, this property repackages the
    /// WidgetTest property as a list containing a single item. If an
    /// attempt is made to set this property to a list containing more than
    /// one item, an exception will be thrown. But why bother? Just use the
    /// WidgetTest property.
    /// </summary>
    private IList<WidgetTest> _widgetTests;
    protected virtual IList<WidgetTest> WidgetTests
    {
        get
        {
            if (_widgetTests.Count == 0 && WidgetTest != null)
            {
                _widgetTests.Add(WidgetTest);
            } 
            else if (_widgetTests.Count > 0 && WidgetTest == null)
            {
                _widgetTests.Clear();
            } 
            else if (_widgetTests.Count > 0 && WidgetTest != _widgetTests[0])
            {
                _widgetTests.Clear();
                _widgetTests.Add(WidgetTest);
            }
            return _widgetTests;
        }
        set
        {
            if (value != null && value.Count > 1)
            {
                throw new Exception("The WidgetTest collection may not contain more than one item.");
            }
            if (value != null && value.Count == 1)
            {
                WidgetTest = value[0];
            }
            else
            {
                WidgetTest = null;
            }

            //Store the reference
            _widgetTests = value;
        }
    }
}

  <class name="Widget" table="widgets">
    ...
    <id name="Id" type="Guid" column="widgetId">
      ...
    </id>
    ...                 
    <bag name="WidgetTests" inverse="true" cascade="all-delete-orphan" access="property">
      ...
      <key column="widgetId" />
      <one-to-many class="WidgetTest" />
    </bag>

  </class>

增强的灵感:

http://www.onkarjoshi.com/blog/188/hibernateexception-a-collection-with-cascade-all-delete-orphan-was-no-longer-referenced-by-the-owning-entity-instance/comment-page-1/