如何在 NHibernate 中不重复加载关联? [英] How to Eager Load Associations without duplication in NHibernate?

问题描述

我需要加载一个包含这么多孩子和孩子的孩子的非常大的对象列表.最好的方法是什么?

I'd need to load a list of very large objects with so many children and children of children. what's the best approach to take?

我正在使用 Oracle 11g 数据库并且我已经编写了以下方法，但它导致笛卡尔积(重复结果):

I'm using Oracle 11g database and I've written the below method but it results in cartesian product (duplicated results):

 public IList<ARNomination> GetByEventId(long eventId)
        {
            var session = this._sessionFactory.Session;

            var nominationQuery = session.Query<ARNomination>().Where(n => n.Event.Id == eventId);

            using (var trans = session.Transaction)
            {
                trans.Begin();

                // this will load the Contacts in one statement
                nominationQuery
                    .FetchMany(n => n.Contacts)
                    .ToFuture();

                // this will load the CustomAttributes in one statement
                nominationQuery
                    .FetchMany(n => n.CustomAttributes)
                    .ToFuture();

                // this will load the nominations but joins those two tables in one statement which results in cartesian product
                nominationQuery
                    .FetchMany(n => n.CustomAttributes)
                    .FetchMany(n => n.Contacts)
                    .ToFuture();

                trans.Commit();
            }

            return nominationQuery.ToList();
        }

推荐答案

获取集合是一个困难的操作.它有很多副作用(正如您所意识到的，当获取更多集合时).但即使获取一个集合，我们也会加载许多重复的行.

Fetching Collections is a difficult operation. It has many side effects (as you realized, when there are fetched more collections). But even with fetching one collection, we are loading many duplicated rows.

一般来说，对于集合加载，我建议使用批处理.这将执行更多的 SQL 查询......但不是那么多，更重要的是，您可以在根列表 ARNomination 上进行分页.

In general, for collections loading, I would suggest to use the batch processing. This will execute more SQL queries... but not so much, and what is more important, you can do paging on the root list ARNomination.

参见:19.1.5.使用批量提取，您可以找到更多详细信息.

See: 19.1.5. Using batch fetching you can find more details.

您必须使用属性 batch-szie="25" 标记您的集合和/或实体.

You have to mark your collections and/or entities with an attribute batch-szie="25".

xml:

<bag name="Contacts" ... batch-size="25">
...

流利:

HasMany(x => x.Contacts)
  ...
  .BatchSize(25)

请在这里检查几个参数:

Please, check few arguments here:

• NHibernate QueryOver with Fetch 导致多个 sql 查询和数据库命中
• 这是在 NHibernate 中预先加载子集合的正确方法吗
• https://stackoverflow.com/q/18419988/1679310

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