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I know the basic ways to avoid the N+1 selects problem in Hibernate/NHibernate, but have run into a variant of the problem that I can't find a good solution to.

I have the following three entities mapped: Item, Category and Customer. An item is associated many-to-many to Category, and a Category is mapped many-to-one to Customer. So far, nothing special.

A standard query in my application is to get all the items for a given customer. I do this using the following criteria, trying to fetch the items' categories eagerly, to avoid N+1 selects when examining the items' Categories property:

ICriteria criteria = mySession.CreateCriteria(typeof(Item));
    .CreateCriteria("Categories", NHibernate.SqlCommand.JoinType.InnerJoin)
        .Add(Expression.Eq("Customer", c));
criteria.SetFetchMode("Categories", FetchMode.Eager);

return criteria.List();

However, this doesn't work, NHibernate still fetches the categories with one select per item later on.

What I believe is going on is that NHibernate knows that the result from the first query is filtered (on Customer), and that the categories returned by the query might not be complete, hence it later has to do a separate query to get the categories. (Is this assumption correct? It seems reasonable to me that NHibernate must work this way to ensure correct results.)

However, according to my business rules (or what you want to call them), an item can't belong to categories from more than one customer, so in reality I know the result from the first query is actually complete.

My question is: can I tell NHibernate about this business rule in any way? Is there another way to avoid N+1 selects in this type of situation (which seems pretty common)?

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1 回答 1

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将尝试回答我自己的问题，因为到目前为止我还没有得到任何答案。

我的解决方案是将问题分为两个查询：首先获取属于相关客户的商品的 ID：

IQuery query = mySession.CreateQuery("select item.Id from Item as item "
    + "join item.Categories as category "
    + "join category.Customer customer "
    + "where customer.id=:id")
    .SetInt32("id", c.Id);
IList itemIds = query.List();

然后，我只使用 ID 就可以在不涉及任何客户限制的情况下获取实际项目。这样，NHibernate 知道它可以从单个连接中获取所有类别，避免了问题中提到的 N+1 选择：

ICriteria criteria = mySession.CreateCriteria(typeof(MapItem))
    .SetFetchMode("Categories", FetchMode.Eager)
    .SetResultTransformer(new DistinctRootEntityResultTransformer())
    .Add(Expression.In("Id", itemIds));
IList items = criteria.List();

我想不出任何解决方案来将其简化为一个有效的单一查询。此外，这种方法迫使程序员对 NHibernate 的内部工作了解得太多，在编写新的查询或条件时很容易错过。更通用的解决方案将是可取的。

于 2009-06-23T12:37:16.487 回答