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Dealing with subqueries in LINQ-to-NHibernate

توسط در چهارشنبه, مارس 3, 2010
Some days ago I was in a situation that needed to use sub queries in LINQ-to-NHibernate. But as LINQ-to-NHibernate does not support sub queries (because of Criteria does not support it) I asked help from StackOverflow. The best response was that it’s better to use HQL and forget about LINQ-to-NHibernate in the case of subqueries. Unfortunately that wasn’t a good solution for me, because we were hoping using LINQ-to-NHibernate, we will get rid of weakly typed HQL queries.


In order to find a work around, my colleague Masoud Ramezani suggested to use 2 LINQ queries to get results. One LINQ-to-NHibernate query for no sub query related section of the query and a second LINQ-to-Object query for sections that contain sub queries. Following 2 snippets shows complete query and divided query that act as a work around to sub query problem. 

<br />internal List<Letter> SearchLetter(DateTime? fromDate, DateTime? toDate, string receiverOrganizationSearch, SendingInformationDAO sendingInformationDAO)<br />{<br />List<Letter> list = null;<br />var q = from l in session.Linq<Letter>()<br />    where<br />    l.Sec.ID == currentSec.ID &&<br />    (l.Date >= fromDate && l.Date <= toDate)  &&<br />    Yas1(sendingInformationDAO.RetrieveReceivers(l), receiverOrganizationSearch)<br />    select l;<br />return q.ToList<Letter>();<br />}<br />





second code:

<br />internal List<Letter> SearchLetter(DateTime? fromDate, DateTime? toDate, string receiverOrganizationSearch, SendingInformationDAO sendingInformationDAO)<br />{<br />  List<Letter> list = null;<br />  var q = from l in session.Linq<Letter>()<br />    where<br />    l.Sec.ID == currentSec.ID &&<br />    (l.Date >= fromDate && l.Date <= toDate)<br />    select l;<br /><br />  List<Letter> first = q.ToList<Letter>();<br /><br />  var q2 = from l in first<br />           select l;<br /><br />  if (receiverOrganizationSearch != null)<br />    q2 = q2.Where(l => Yas1(sendingInformationDAO.RetrieveReceivers(l), receiverOrganizationSearch));<br /><br />  return q2.ToList<Letter>();<br />}<br />





Yas1 method:

<br />private bool Yas1(EntityCollection<SendingInformation> collection, string org_name)<br />{<br />if (collection == null)<br />    return false;<br /><br />    org_name = org_name.Trim();<br /><br />    foreach (SendingInformation item in collection)<br />    if (item != null && item.To != null && item.To.Organization != null &&<br />                    item.To.Organization.Name != null && item.To.Organization.Name.Contains(org_name))<br />    return true;<br /><br />    return false;<br />}<br />



Please consider this approach works only if you use dynamic queries. If someone puts Yas1 directly in LINQ-to-Object query, that will not work as expected.

UPDATE (11/15/2010) :


LINQ-to-NHibernate does support sub-queries in where clause. For more info go to this and this link. There is also limitations.

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