Nhibernate -与Cascade all-delete-orphan的一对一映射，不删除孤儿

我有一个'Interview'实体，它与'FormSubmission'实体有一对一的映射，可以说，Interview实体是主导方，映射是:

<class name="Interview">
    <id name="Id" column="Id" type="Int64">
        <generator class="identity" />
    </id>
    // other props (snip)....
    <one-to-one name="Submission" class="FormSubmission"
        cascade="all-delete-orphan" />
</class>
<class name="FormSubmission">
    <id name="Id" column="Id" type="Int64">
        <generator class="foreign">
            <param name="property">Interview</param>
        </generator>
    </id>
    // other props (snip)....
    <one-to-one name="Interview" class="Interview"
        constrained="true" cascade="none" />
</class>

两个实体都是聚合的一部分，面谈作为聚合根。我试图通过面试实体保存/更新/删除表单提交，因此我将关联的面试结束映射为cascade="all-delete-orphan"。例如，我可以像这样创建一个新的FormSubmission:

myInterview.Submission = new FormSubmission(myInterview);
InterviewRepository.Save(myInterview);

…这工作得很好，FormSubmission被保存了。然而，我似乎无法删除FormSubmission，我正试图这样做:

myInterview.Submission = null;
InterviewRepository.Save(myInterview);

…但这似乎并没有删除FormSubmission。我已经尝试将null赋值给关联的两边:

myInterview.Submission.Interview = null;
myInterview.Submission = null;
InterviewRepository.Save(myInterview);

我甚至尝试在FormSubmission端设置cascade="all-delete-orphan"，但似乎没有任何效果。我错过了什么?

<class name="Interview">
    <id name="Id" column="Id" type="Int64">
        <generator class="identity" />
    </id>
    <property name="Name" />
    <many-to-one name="Submission" unique="true" cascade="all-delete-orphan" />
</class>
<class name="FormSubmission">
    <id name="Id" column="Id" type="Int64">
        <generator class="identity" />
    </id>
    <property name="Name" />
    <one-to-one name="Interview" cascade="all-delete-orphan" property-ref="Submission"  />
</class>