Fluent NHibernate: LEFT OUTER JOIN ... AND Query

Question

I have the next Fluent NHibernate query using QueryOver:

IQueryOver<Task> query = session
    .QueryOver<Task>()
    .JoinAlias(x => x.Subject, () => subject, JoinType.LeftOuterJoin)
    .JoinAlias(() => subject.Localizations, () => localization, JoinType.LeftOuterJoin)
    .Where(() => localization.Language.Id == languageId || localization.Language.Id == null);
    .Where(x => x.UserId == userId)

This performs the next query:

SELECT * FROM Tasks left outer join Subjects on Tasks.SubjectId = Subjects.IdAsignatura 
left outer join Languages on Subjects.SubjectId=Languages.SubjectId
WHERE Tasks.UserId = xxx 
and (Languages.LanguageId = yyy or Languages.LanguageId = is null)

I would like to avoid the last line using "LEFT OUTER JOIN ... AND" like this:

SELECT * FROM Tasks left outer join Subjects on Tasks.SubjectId = Subjects.IdAsignatura 
left outer join Languages on Subjects.SubjectId=Languages.SubjectId AND Languages.LanguageId = yyy
WHERE Tasks.UserId = xxx

Is there a way to perform "LEFT OUTER JOIN ... AND" Fluent NHibernate queries?

Radim Köhler Radim Köhler · Accepted Answer · 2014-09-29T13:24:38

There is one overload of the CreateAlias:

/// <param name="path">Lambda expression returning association path</param>
/// <param name="alias">Lambda expression returning alias reference</param>
/// <param name="joinType">Type of join</param>
/// <param name="withClause">Additional criterion for the SQL on clause</param>
/// <returns>
/// criteria instance
/// </returns>
IQueryOver<TRoot, TSubType> JoinAlias<U>(Expression<Func<U>> path
     , Expression<Func<U>> alias, JoinType joinType
     , ICriterion withClause);

That means, that we can pass the Restriction from WHRE into ON clause

/// instead of this
.JoinAlias(() => subject.Localizations, () => localization, JoinType.LeftOuterJoin)
.Where(() => localization.Language.Id == languageId 
          || localization.Language.Id == null);

/// We can use this:
.JoinAlias(() => subject.Localizations, () => localization, JoinType.LeftOuterJoin
            // the 4th param is appended to ON clause with AND operator
            , Restrictions.Where(() => localization.Language.Id == languageId)
)

The withClause could be injected into the ON part of our SELECT, but always with AND operator. So it can only apply more restrictions than the basic mapped relation.

Fluent NHibernate: LEFT OUTER JOIN ... AND Query

1 Answers