I have an NHibernate query that looks like this:
var query = Session.CreateQuery(#"
select o
from Order o
left join o.Products p
where
(o.CompanyId = :companyId) AND
(p.Status = :processing)
order by o.UpdatedOn desc")
.SetParameter("companyId", companyId)
.SetParameter("processing", Status.Processing)
.SetResultTransformer(Transformers.DistinctRootEntity);
var data = query.List<Order>();
I want to implement paging for this query, so I only return x rows instead of the entire result set.
I know about SetMaxResults() and SetFirstResult(), but because of the left join and DistinctRootEntity, that could return less than x Orders.
I tried "select distinct o" as well, but the sql that is generated for that (using the sqlserver 2008 dialect) seems to ignore the distinct for pages after the first one (I think this is the problem).
What is the best way to accomplish this?
In these cases, it's best to do it in two queries instead of one:
Load a page of orders, without joins
Load those orders with their products, using the in operator
There's a slightly more complex example at http://ayende.com/Blog/archive/2010/01/16/eagerly-loading-entity-associations-efficiently-with-nhibernate.aspx
Use SetResultTransformer(Transformers.AliasToBean()) and get the data that is not the entity.
The other solution is that you change the query.
As I see you're returning Orders that have products which are processing.
So you could use exists statement. Check nhibernate manual at 13.11. Subqueries.
Related
How can I create the following SQL statement in Nhibernate using CreateCriteria:
SELECT distinct top 20 a.* from ActivityLog a
left join WallPost w on a.ActivityLogId = w.ActivityLogId left join ItemStatus i on i.StatusId = w.ItemStatus
I always tend to get all columns from all tables returned in the sql statement producing duplicates even though I map it to the ActivityLog table. I am also doing paging as the code below shows:
ICriteria crit = nhelper.NHibernateSession.CreateCriteria(typeof(Model.ActivityLog), "a").CreateAlias("a.WallPosts", "w",CriteriaSpecification.LeftJoin)
.CreateAlias("w.ItemStatus", "i", CriteriaSpecification.LeftJoin)
.SetMaxResults(pageSize).SetFirstResult(startRow).AddOrder(Order.Desc("a.Date"));
Thanks
H
Sounds like you have set lazy loading to false in your mapping files meaning that all the associations and child collections are being loaded up too. Can you verify that?
You ask "How to select one table using NHibernate CreateQuery" (HQL). In that case you can choose what to fetch using select.
In your text you're using criteria. AFAIK, you cannot directly control what columns you want to read. However, if you create a DetachedCriteria for the join, this won't be fetched.
The diagram above is a simplified version of the database structure that I use to log item locations through time. I wrote the following SQL query which returns the current item inventory of each location:
select *
from ItemLocationLog l
inner join
(select g.idItemLocationLog, max(g.dateTime) as latest
from ItemLocationLog g
group by g.idItem)
as i
on l.idItem = i.idItem and l.dateTime = i.latest
The problem I'm having is that I want to convert that to HQL, but I haven't found the syntax to perform an INNER JOIN on a subquery, and it seems like this is not supported. Is there a way to convert the above to HQL (or a Criteria) or will I have to use a standard SQL query in this case? Thanks.
http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/queryhql.html#queryhql-subqueries
Note that HQL subqueries can occur only in the select or where clauses.
You can rewrite the query so that the subquery is part of the where clause instead. Referencing the l.idItem in the subquery
I have one Linq to SQL query that simply joins three tables and then performs grouping. Here is the query:
(from s in mktActualSales
join p in sysPeriods on s.PeriodID equals p.PeriodID
join d in setupDesignations on s.PositionID equals d.DesignationID
group new { s, p } by new{d.Title,d.DesignationID} into temping
select
new
{
SPOPosition = temping.Key.Title,
SalesPeriods = temping.Select(x=>new {PeriodID = x.p.PeriodID, StartDate = x.p.StartDate, EndDate = x.p.EndDate, SaleTargetID = x.s.ActualSaleID, IsApproved = x.s.IsApproved}),
PositionID = temping.Key.DesignationID
}).Take(5)
When I inspect SQL generated (executed) by this query in LinqPad, there are 6 SQL statements; first one performs join and grouping and rest of queries are the same just calling same query over and over again with different parameters. Obviously parameters are the values included in Key of the group.
It makes me believe that for a record with 130 groups Linq will hit the database 131 times. How can I save so much hits to the database? Should I perform grouping after loading data into memory i.e after calling ToList on joined query?
(I am assuming mktActualSales is your table and you missed putting the Datacontext in the question.)
If it is your table you should have a look at LoadOptions on your DataContext. With this you can eager load some of the related data and this might prevent the repetitive queries.
http://msdn.microsoft.com/en-us/library/system.data.linq.dataloadoptions.aspx
If it is not a table, have a look on how you populate mktActualSales but judging from your question I assume it is a table.
I'm trying to express a SQL query using NHibernate's Criteria API, and I'm running into difficulty because I'm thinking in a database-centric way while NHibernate is object-centric.
SQL (works great):
select outerT.id, outerT.col1, outerT.col2, outerT.col3
from tbl outerT
inner join
(select max(innerT.id)
from tbl innerT
group by innerT.col1) grpT
on outerT.id = grpT.id
Essentially, this is a self-join of a table against a subset of itself. I suppose I could try turning the self-join into a restriction:
select outerT.id, outerT.col1, outerT.col2, outerT.col3
from tbl outerT
where outerT.id in (select max(innerT.id) from tbl innerT group by innerT.col1)
But I'm not sure how to express that using NHibernate either; I'm fighting with the DetachedCriteria's ProjectionList and wanting to select only max(id) while grouping by col1.
Thanks so much for your suggestions!
I don't know if I should post this as a new answer or to add it as a comment on the original question, but I think I've resolved a similar problem in this thread:
Selecting on Sub Queries in NHibernate with Critieria API
AFAIK you cannot join to subqueries at all in NHibernate but you can re-organise the query to use either an EXISTS or IN clause to replicate the same functionality.
I realise the question asks for this to be done using the Criteria API but I thought I would post a HQL version which may give someone else some ideas.
var results = session.CreateQuery("from Product p where p.Id in (
select max(p2.id)
from Product p2
group by p2.col1
)")
I also found this JIRA issue surrounding the Criteria API and not including group by columns in the select. Currently it looks like what you want cannot be achieved using the Criteria API at all.
Group By Property without adding it to the select clause
UPDATE
Using the example from Monkey Coders post looks like you can do this:
var subquery = DetachedCriteria.For<Product>("p")
.SetProjection(Projections.ProjectionList()
.Add(Projections.GroupProperty("p.Col1"))
.Add(Restrictions.EqProperty("p2.Id", Projections.Max("p.Id"));
var query = DetachedCriteria.For<Product>("p2")
.Add(Subqueries.Exists(subquery));
Which would produce the following SQL
select *
from Product p2
where exists (
select p.col1
from Product p
group by p.col1
having p2.Id=max(p.Id)
)
I have two tables in a SQLite DB, INVITEM and SHOPITEM. Their shared attribute is ItemId and I want to perform an INNER JOIN. Here's the query:
SELECT INVITEM.CharId AS CharId,
INVITEM.ItemId AS ItemId
FROM (INVITEM as INVITEM
INNER JOIN SHOPITEM AS SHOPITEM
ON SHOPITEM.ItemId = INVITEM.ItemId)
WHERE ItemId = 3;
SQLite doesn't like it :
SQL error: ambiguous column name: ItemId
The error goes away if I write WHERE INVITEM.ItemId = 3, but since the WHERE condition is more or less user-specified, I rather make it work without having to specify the table. NATURAL JOIN seems to solve the issue, but I'm not sure if the solution is general enough (ie I could use in this case, but I'm not sure if I can use in every case)
Any alternate SQL syntax that would fix the problem?
I would write this query this way:
SELECT i.CharId AS CharId, i.ItemId AS ItemId
FROM INVITEM as i INNER JOIN SHOPITEM AS s USING (ItemId)
WHERE i.ItemId = 3;
I'm using the USING (ItemId) syntax which is just a matter of taste. It's equivalent to ON (i.ItemID = s.ItemID).
But I resolved the ambiguity by qualifying i.ItemID in the WHERE clause. You would think this is unnecessary, since i.ItemID = s.ItemID. They're both equal by commutativity, so there's no semantic ambiguity. But apparently SQLite isn't smart enough to know that.
I don't like to use NATURAL JOIN. It's equivalent to an equi-join of every column that exists in both tables with the same name. I don't like to use this because I don't want it to compare columns that I don't want it to, simply because they have the same name.
I would steer clear of allowing the user to write SQL clauses directly. This is the source of SQL Injection vulnerabilities.
If you need the query to be flexible, try parsing the user's input and adding the appropriate where clause.
Here is some C# code to show the general idea
// from user input
string user_column = "ItemID";
string user_value = "3";
string sql = "SELECT INVITEM.CharId AS CharId, INVITEM.ItemId AS ItemId FROM (INVITEM as INVITEM INNER JOIN SHOPITEM AS SHOPITEM ON SHOPITEM.ItemId = INVITEM.ItemId) ";
if (user_column == "ItemID")
{
// using Int32.Parse here to prevent rubbish like "0 OR 1=1; --" being entered.
sql += string.Format("WHERE INVITEM.ItemID={0}",Int32.Parse(user_value));
}
Obviously if you're dealing with more than one clause, you'd have to substitute AND for WHERE in subsequent clauses.
Just change your column alias to something similar, but unique (such as ITEM_ID).