NHibernate QueryOver: How to join unrelated entities?

Question

I have the following query working which gets the results I want:

int associatedId = 123;

MyObject alias = null;

var subQuery = QueryOver.Of<DatabaseView>()
    .Where(view => view.AssociatedId == associatedId)
    .And(view => view.ObjectId == alias.ObjectId)
    .Select(view => view.ObjectId);

var results = session.QueryOver<MyObject>(() => alias)
    .WithSubquery.WhereExists(subQuery)
    .List();

The DatabaseView has been mapped as an actual NHibernate entity (so I can use it with QueryOver), but it is not associated to MyObject in the HBM mappings.

This query returns an IList<MyObject> using a SELECT ... FROM MyObject WHERE EXISTS (subquery for DatabaseView here). How can I re-write this to return the same data but using a JOIN instead of sub query?

Answer 1

In NHibernate 5.1+ it's possible for QueryOver/Criteria via Entity Join:

int associatedId = 123;

MyObject alias = null;
DatabaseView viewAlias = null;

var results = session.QueryOver<MyObject>(() => alias)
    .JoinEntityAlias(() => viewAlias, () => viewAlias.ObjectId == alias.ObjectId && viewAlias.AssociatedId == associatedId)
    .List();

Criteria example:

int associatedId = 123;
var results = session.CreateCriteria<MyObject>("alias")
    .CreateEntityAlias(
            "viewAlias",
            Restrictions.EqProperty("viewAlias.ObjectId", "alias.ObjectId")
            && Restrictions.Eq("viewAlias.AssociationId", associatedId),
            JoinType.InnerJoin,
            typeof(DatabaseView).FullName)
    .List();