I am simply trying to write a query that will look like this and be eager loaded:
Select * from Users where Id IN (1,2,3)
Select * from Bids where UserId in (1,2,3)
Right now it's causing problems because the join is returning too many results.
Try using multicriteria (or Future as in this example), should issue 2 queries in 1 batch and give you the eager loading:
var bids = Session.QueryOver<Bid>()
.JoinQueryOver(b => b.User)
.WhereRestrictionOn(u => u.Id).IsIn(ids)
.Future<Bid>();
var users = Session.QueryOver<User>()
.WhereRestrictionOn(u => u.Id).IsIn(ids)
.List<User>();
The only problem is it will join Bid -> User, to avoid that you could use HQL:
var bids = Session.CreateQuery("from Bid b where b.User.id in (:userIds)")
.SetParameterList("userIds", ids)
.Future<Bid>();
var users = Session.QueryOver<User>()
.WhereRestrictionOn(u => u.Id).IsIn(ids)
.List<User>();
Related
I have a SQL Query which I want to convert in Linq or want to show data
as pictured here
Here is the query:
select sae_subcategorymaster.subject, count(sae_tblbookdetail.title)
from sae_tblbookdetail inner join sae_subcategorymaster
on sae_subcategorymaster.subject=sae_tblbookdetail.subject
group by sae_subcategorymaster.subject
What is a simple way to do this?
Grouping is supported in LinqEF. Provided you have your entities related. (Books have a reference to their subcategory.)
var totals = context.Books
.GroupBy(book => book.SubCategory.Subject)
.Select(group => new
{
Subject = group.Key,
BookCount = group.Count()
}).ToList();
SQL query is:
select B.* from A inner join B on A.b_id = B.id where A.x in (1,2,3)
A <-> B relation is many-to-one
I need to filter by A but fetch related B.
UPDATE:
I tried this NH QueryOver
Session.QueryOver<A>.Where(a => a.x.IsIn(array)).JoinQueryOver(a => a.B).Select(a => a.B).List<B>()
but it results in a N+1 sequence of queries: the first one fetches IDs of related Bs, and others fetch related Bs one by one by ID (analyzed via NHProf). I want it to fetch a list of Bs in one go.
UPDATE 2:
for now I worked around this with subquery
Session.QueryOver(() => b).WithSubquery.WhereExists(QueryOver.Of<A>().Where(a => a.x.IsIn(array)).And(a => a.b_id == b.id).Select(a => a.id)).List<B>()
but I still hope to see an example of QueryOver without subquery as I tend to think subquery is less efficient.
This works (at least in my test application):
var list = session.QueryOver<A>()
.Where(a => a.X.IsIn(array))
.Fetch(x => x.B).Eager
.List<A>()
.Select(x => x.B);
Note that the .Select() statement is a normal LINQ statement, not part of NHibernate.
Generated SQL:
SELECT
this_.Id as Id0_1_,
this_.B as B0_1_,
this_.X as X0_1_,
b2_.Id as Id1_0_,
b2_.SomeValue as SomeValue1_0_
FROM A this_ left outer join B b2_ on this_.B=b2_.Id
WHERE this_.X in (?, ?, ?)
It's not optimal if A is a very large class, of course.
An NHibernate-only solution with a subquery:
var candidates = QueryOver.Of<A>()
.Where(a => a.X.IsIn(array))
.Select(x => x.B.Id);
var list = session.QueryOver<B>()
.WithSubquery.WhereProperty(x => x.Id).In(candidates).List();
I'll try to find the reason why the most obvious solution (just adding Fetch().Eager) doesn't work as expected. Stay tuned!
I'm trying to get something similar to the SQL below via QueryOver:
SELECT
docs.*,
(SELECT TOP 1 eventDate from events WHERE id=docs.id
AND type=4 ORDER BY eventDate DESC) as eventDate
FROM documents as docs
WHERE doc.accountId = ...
I've got close with a projection, however I'm not sure how to get the entire documents table back. Documents has a one-to-many relationship with Events, I don't want to outer join as it will bring multiple results, and an inner join may not bring back a row:
var query = QueryOver<Document>
.Where(d => d.Account == account)
.SelectList(list => list
.Select(d => d)
.Select(d => d.Events.OrderByDescending(e => e.EventDate).FirstOrDefault(e => e.Type == 4))
)
.List<object[]>()
.Select(d => return new DocumentSummary(d[0],d[1]) etc.);
Is there an easier way of performing subqueries for columns? I'm reluctant to replace this with the property performing a query in its get.
After some research it looks like HQL (which QueryOver is converted into) does not support TOP inside subqueries.
My solution: create a view which includes the computed properties, and then mark these properties in the mappings files as insert="false" and update="false"
Imagine the following (simplified) database layout:
We have many "holiday" records that relate to going to a particular Accommodation on a certain date etc.
I would like to pull from the database the "best" holiday going to each accommodation (i.e. lowest price), given a set of search criteria (e.g. duration, departure airport etc).
There will be multiple records with the same price, so then we need to choose by offer saving (descending), then by departure date ascending.
I can write SQL to do this that looks like this (I'm not saying this is necessarily the most optimal way):
SELECT *
FROM Holiday h1 INNER JOIN (
SELECT h2.HolidayID,
h2.AccommodationID,
ROW_NUMBER() OVER (
PARTITION BY h2.AccommodationID
ORDER BY OfferSaving DESC
) AS RowNum
FROM Holiday h2 INNER JOIN (
SELECT AccommodationID,
MIN(price) as MinPrice
FROM Holiday
WHERE TradeNameID = 58001
/*** Other Criteria Here ***/
GROUP BY AccommodationID
) mp
ON mp.AccommodationID = h2.AccommodationID
AND mp.MinPrice = h2.price
WHERE TradeNameID = 58001
/*** Other Criteria Here ***/
) x on h1.HolidayID = x.HolidayID and x.RowNum = 1
As you can see, this uses a subquery within another subquery.
However, for several reasons my preference would be to achieve this same result in NHibernate.
Ideally, this would be done with QueryOver - the reason being that I build up the search criteria dynamically and this is much easier with QueryOver's fluent interface. (I had started out hoping to use NHibernate Linq, but unfortunately it's not mature enough).
After a lot of effort (being a relative newbie to NHibernate) I was able to re-create the very inner query that fetches all accommodations and their min price.
public IEnumerable<HolidaySearchDataDto> CriteriaFindAccommodationFromPricesForOffers(IEnumerable<IHolidayFilter<PackageHoliday>> filters, int skip, int take, out bool hasMore)
{
IQueryOver<PackageHoliday, PackageHoliday> queryable = NHibernateSession.CurrentFor(NHibernateSession.DefaultFactoryKey).QueryOver<PackageHoliday>();
queryable = queryable.Where(h => h.TradeNameId == website.TradeNameID);
var accommodation = Null<Accommodation>();
var accommodationUnit = Null<AccommodationUnit>();
var dto = Null<HolidaySearchDataDto>();
// Apply search criteria
foreach (var filter in filters)
queryable = filter.ApplyFilter(queryable, accommodationUnit, accommodation);
var query1 = queryable
.JoinQueryOver(h => h.AccommodationUnit, () => accommodationUnit)
.JoinQueryOver(h => h.Accommodation, () => accommodation)
.SelectList(hols => hols
.SelectGroup(() => accommodation.Id).WithAlias(() => dto.AccommodationId)
.SelectMin(h => h.Price).WithAlias(() => dto.Price)
);
var list = query1.OrderByAlias(() => dto.Price).Asc
.Skip(skip).Take(take+1)
.Cacheable().CacheMode(CacheMode.Normal).List<object[]>();
// Cacheing doesn't work this way...
/*.TransformUsing(Transformers.AliasToBean<HolidaySearchDataDto>())
.Cacheable().CacheMode(CacheMode.Normal).List<HolidaySearchDataDto>();*/
hasMore = list.Count() == take;
var dtos = list.Take(take).Select(h => new HolidaySearchDataDto
{
AccommodationId = (string)h[0],
Price = (decimal)h[1],
});
return dtos;
}
So my question is...
Any ideas on how to achieve what I want using QueryOver, or if necessary Criteria API?
I'd prefer not to use HQL but if it is necessary than I'm willing to see how it can be done with that too (it makes it harder (or more messy) to build up the search criteria though).
If this just isn't doable using NHibernate, then I could use a SQL query. In which case, my question is can the SQL be improved/optimised?
I have manage to achieve such dynamic search criterion by using Criteria API's. Problem I ran into was duplicates with inner and outer joins and especially related to sorting and pagination, and I had to resort to using 2 queries, 1st query for restriction and using the result of 1st query as 'in' clause in 2nd creteria.
my code is:
List<Benutzer> users = (from a in dc.Benutzer
select a).ToList();
I need this code but I only want to select 3 of the 20 Columns in the "Benutzer"-Table.
What is the syntax for that?
Here's a query expression:
var users = (from a in dc.Benutzer
select new { a.Name, a.Age, a.Occupation }).ToList();
Or in dot notation:
var users = dc.Benutzer.Select(a => new { a.Name, a.Age, a.Occupation })
.ToList();
Note that this returns a list of an anonymous type rather than instances of Benutzer. Personally I prefer this approach over creating a list of partially populated instances, as then anyone dealing with the partial instances needs to check whether they came from to find out what will really be there.
EDIT: If you really want to build instances of Benutzer, and LINQ isn't letting you do so in a query (I'm not sure why) you could always do:
List<Benutzer> users = dc.Benutzer
.Select(a => new { a.Name, a.Age, a.Occupation })
.AsEnumerable() // Forces the rest of the query to execute locally
.Select(x => new Benutzer { Name = x.Name, Age = x.Age,
Occupation = x.Occupation })
.ToList();
i.e. use the anonymous type just as a DTO. Note that the returned Benutzer objects won't be associated with a context though.
List<Benutzer> users = (from a in dc.Benutzer
select new Benutzer{
myCol= a.myCol,
myCol2 = a.myCol2
}).ToList();
I think that's what you want if you want to make the same kind of list. But that assumes that the properties you are setting have public setters.
try:
var list = (from a in dc.Benutzer select new {a.Col1, a.Col2, a.Col3}).ToList();
but now you have list of anonymous object not of Benutzer objects.