NHibernate Linq Provider eager fetching lazy string property

Question

I am considered new to NHibernate. I want my Article entity class to have two lazy properties. One of them is User other is Content. My entity and mapping is like

<class name="Article" table="TBL_ARTICLE">
    <id name="Id" column="ART_ID">
        <generator class="native" />
    </id>
    <property name="UserId" column="USR_ID" not-null="true" />
    <many-to-one name="User" column="USR_ID" insert="false" update="false" />
    <property name="Content" column="ART_CONTENT" not-null="true" lazy="true" />
</class>

public class Article
{
    public virtual long Id { get; set; }
    public virtual long UserId { get; set; }
    public virtual User User { get; set; }
    public virtual string Content { get; set; }
}

I select it like

using(ISession session = sessionFactory.OpenSession()) {
     return session.Query<Article>()
                   .SingleOrDefault(a => a.Id == id);
}

It works fine. When I try to access User or Content property outside the using block I get some kind of lazy loading exception. This is what I've expected.

For some cases I like to fetch User data eagerly. I select it like:

using(ISession session = sessionFactory.OpenSession()) {
     return session.Query<Article>()
                   .Fetch(a => a.User)
                   .SingleOrDefault(a => a.Id == id);
}

It still works fine. When I try to access User property outside the using block I can have its property values but when try to access Content property I stil get some kind of lazy loading exception and this is still what I've expected.

When I want to fetch content data like

using(ISession session = sessionFactory.OpenSession()) {
     return session.Query<Article>()
                   .Fetch(a => a.Content)
                   .SingleOrDefault(a => a.Id == id);
}

I get an exception:

Invalid join: a.Content
[.SingleOrDefault[Repository.NH.Article](.Fetch[Repository.NH.Article,System.String]
(NHibernate.Linq.NhQueryable`1[Repository.NH.Article], Quote((a, ) => (a.Content)), ),
Quote((a, ) => (Equal(a.Id, 1))), )]

I read some blog post about doing this by HQL but I am looking for a solution using Linq Provider.

connectedsoftware connectedsoftware · Accepted Answer · 2013-06-04T14:01:12

The Fetch construct only works on properties that map relationships between objects, so properties that are mapped with "References", "Many-To-One" or "Many-To-Many".

This is why it works as expected with the User property but not against the string Content property.

After further investigation I don't think you can achieve what you want using LINQ syntax but it should be possible using an equivalent HQL statement using the fetch all properties hint:

session.CreateQuery("from Article fetch all properties")

NHibernate Linq Provider eager fetching lazy string property

3 Answers