Question

I’m working on a .NET C# project on which I had to use NHibernate Mapping Attributes to map my objects to my tables in my database.

Now let’s explain what my problem is.

I have two mapped classes, for example ClassA and ClassB. In my database, table A contains a foreign key referencing the primary key of table B. Hence, I have added to ClassA an instance of ClassB mapped in many-to-one:

private ClassB b;
[ManyToOne(0, Name = "B", Column = "ID_TABLE_B_TABLE_A", Class = "ClassB", Update = false, Insert = false)]
public virtual ClassB B 
{ 
    get { return b; } 
    set { b= value; } 
}

Now, I want to check the value of a field of ClassB when I’m accessing ClassA. I write the query in HQL:

Session.CreateQuery("select a.Id from ClassA a where a.ClassB.Name = 'xxx' ");

Here’s the generated SQL:

select tablea0_.ID_TABLE_A as col_0_0_ 
from TABLE_A tablea0_, TABLE_B tableb1_
where tablea0_.ID_TABLE_B_TABLE_A = tableb1_.ID_TABLE_B 
and tableb1_.NAME_TABLE_B='xxx'

I thought this kind of HQL query was supposed to generated a join statement rather than a where statement, as I have defined a many-to-one association between the two classes. Something like this would’ve been better:

select tablea0_.ID_TABLE_A as col_0_0_ 
from TABLE_A tablea0_ 
left join TABLE_B tableb1_ on tableb1_.ID_TABLE_B = tablea0_.ID_TABLE_B_TABLE_A 
where tableb1_.NAME_TABLE_B='xxx'

In my opinion, join looks cleaner to where. I would like to know if there is a way to set up the behaviour of NHibernate accordingly, without specifying the join statement explicitly in the HQL query.

Any help would be appreciated !

Answer 1

The join works like this:

Session.CreateQuery("select a.Id from ClassA a join a.ClassB b where b.Name = 'xxx' ");
Session.CreateQuery("select a.Id from ClassA a left join a.ClassB b where b.Name = 'xxx' ");
Session.CreateQuery("select a.Id from ClassA a left outer join a.ClassB b where b.Name = 'xxx' ");

Of course, it’s now for you to figure out which join works best for you. 🙂

More information @ http://docs.jboss.org/hibernate/stable/core/reference/en/html/queryhql.html#queryhql-joins

On a side note, if you want to be one of the cool kids, you could always use Linq to NHibernate:

var result = Session.Linq<ClassA>().Where(a => a.B.Name == 'xxx').ToList();

Would generate an inner join query.