急切加载递归关系 [英] Eagerly load recursive relation

问题描述

我有一个递归的一对多关系，它的默认惰性值为 true.我可以针对 NH API 编写哪些代码来有效地检索整个树，就像我在 SubCategories 映射上设置了 lazy="false" 一样?

I have a recursive one-to-many relationship that has the default lazy value of true. What code can I write against the NH API that will efficiently retrieve the ENTIRE tree AS IF I had lazy="false" on the SubCategories mapping?

这是递归的一对多关系:

Here's the recursive one-to-many relationship:

<class name="Category" lazy="false">
    ...
    <list name="SubCategories" fetch="subselect">
            <key column="ParentCategoryID"/>
            <index column="PositionInList"/>
            <one-to-many class="Category"/>
    </list>

我没有在列表中指定 lazy="false"，因为在我需要运行的大约一半的查询中需要惰性.我在列表中使用了 fetch="subselect" 作为优化，以便我设法检索整个树.

I don't specify lazy="false" on the list since laziness is required in about half the queries I need to run. I have fetch="subselect" on the list as an optimization for when I do manage to retrieve the entire tree.

我已经尝试过 ICriteria API:

I've tried the ICriteria API:

session.CreateCriteria<Category>().SetFetchMode( "SubCategories", FetchMode.Eager ).Add( Restrictions.IsNull("ParentCategory") ).SetResultTransformer( CriteriaSpecification.DistinctRootEntity ).List<Category>();

但那只是急切地加载层次结构中的第一级.

but that only eagerly loaded only the first level in the hierarchy.

推荐答案

参见 Ayende 的站点:有效地选择一棵树.我已经在我自己的应用程序中成功地使用了这种技术.使用 ICriteria，它看起来像这样:

See Ayende's site: Efficiently Selecting a Tree. I have successfully used this technique in my own applications. With ICriteria, it looks like this:

session.CreateCriteria<Category>()
    .SetFetchMode("SubCategories", FetchMode.Join)
    .SetResultTransformer(new DistinctRootEntityResultTransformer())
    .List<Category>()
    .Where(x => x.ParentCategory == null);

此版本与您尝试过的版本之间的主要区别在于ParentCategory == null"过滤器的应用方式.为了检索整个树，它必须被排除在发送到数据库的查询之外——但我们仍然需要查询只返回树的根节点，所以我们将使用 linq 在数据库查询完成.

The main difference between this version and what you tried is how the "ParentCategory == null" filter is applied. It has to be left out of the query that is sent to the database in order to retrieve the whole tree - but we still need the query to only return the root nodes of the tree, so we'll use linq to find those after the database query has completed.

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