NHibernate check for same join in QueryOver - nhibernate

Question

Using QueryOver I am creating a query like this
BulkActionItem bulkActionItemAlias1 = null;
BulkActionItem bulkActionItemAlias2 = null;
var query = GetSession().QueryOver<Student>(() => studentAlias)
.JoinAlias(() => studentAlias.BulkNotifications, () => bulkActionItemAlias1, NHibernate.SqlCommand.JoinType.LeftOuterJoin);
if (query.UnderlyingCriteria.GetCriteriaByAlias("bulkActionItemAlias2") == null
query = query.JoinAlias(() => studentAlias.BulkNotifications, () => bulkActionItemAlias2, NHibernate.SqlCommand.JoinType.LeftOuterJoin);
This will crash because I have the same join twice with different aliases. Is it possible to check if a join already exists on a query, even with a different alias?

Answer 1

I haven't found a built-in way to accomplish this. Typically I use an out parameter with extension methods to keep track of what tables are part of the query. For example:
bool joinedOnBulkNotifications;
BulkNotification notificationAlias = null;
Expression<Func<object>> aliasExpr = () => notificationAlias;
var query = GetSession().QueryOver<Student>(() => studentAlias)
.FilterByBulkNotificationStatus(
someCondition, aliasExpr, out joinedOnBulkNotifications);
public static class QueryExtensions
{
public static IQueryOver<Student, Student> FilterByBulkNotificationStatus(
this IQueryOver<Student, Student> query,
bool someCondition,
Expression<Func<object>> aliasExpr,
out bool joinedOnBulkNotifications)
{
joinedOnBulkNotifications = false;
if (someCondition)
{
joinedOnBulkNotifications = true;
query.JoinAlias(s => s.BulkNotifications, aliasExpr);
}
return query;
}
}
The issue is that you might need to reuse the alias you created later. You might be tempted to pass in a BulkNotification and use that, but this only works if the parameter name matches the name of the variable you pass to the extension method. NHibernate uses the name of the variable to create an alias name, so if these two do not match, you'll get an error. Because of this, you need to wrap the alias in an Expression and use that instead.
This isn't a very clean option, so I hope someone has a better solution.