Does anyone know of a way to do a left outer join with SubSonic 3.0 or another way to approach this problem? What I am trying to accomplish is that I have one table for departments and another table for divisions. A department can have multiple divisions. I need to display a list of departments with the divisions it contains. Getting back a collection of departments which each contain a collection of divisions would be ideal, but I would take a flattened result table too.
Using the LINQ syntax seems to be broken (I am new to LINQ though and may be using it wrong), for example this throws an ArgumentException error:
var allDepartments = from div in Division.All()
join dept in Department.All() on div.DepartmentId equals dept.Id into divdept
select divdept;
So I figured I could fall back to using the SubSonic query syntax. This code however generates an INNER JOIN instead of an OUTER JOIN:
List<Department> allDepartments = new Select()
.From<Department>()
.LeftOuterJoin<Division>(DepartmentsTable.IdColumn, DivisionsTable.DepartmentIdColumn)
.ExecuteTypedList<Department>();
Any help would be appreciated. I am not having much luck with SubSonic 3. I really enjoyed using SubSonic 2 and may go back to that if I can't figure out something as basic as a left join.
Getting back a collection of departments which each contain a collection of divisions would be ideal
SubSonic does this for you (if you setup your relationships correctly in the database), just select all Departments:
var depts = Model.Department.All();
There will be a property in each item of depts named Divisions, which contains a collection of Division objects.
Related
I have recently started working on linq and I was wondering suppose I have 2 related tables Project (<=with fkAccessLevelId) and AccessLevel and I want to just select values from both tables. Now there are 2 ways I can select values from these tables.
The one i commonly use is:
(from P in DataContext.Projects
join AL in DataContext.AccessLevel
on P.AccessLevelId equals AL.AccessLevelId
select new
{
ProjectName = P.Name,
Access = AL.AccessName
}
Another way of doing this would be:
(from P in DataContext.Projects
select new
{
ProjectName = P.Name,
Access = P.AccessLevel.AccessName
}
What i wanted to know is which of these way is efficient if we increase the number of table say 5-6 with 1-2 tables containing thousands of records...?
You should take a look at the SQL generated. You have to understand that there are several main performance bottle necks in a Linq query (in this case I assume a OMG...Linq to SQL?!?!) the usual main bottle neck is the SQL query on the server.
Typically SQL Server has a very good optimizer, so actually, given the same query, refactored, the perf is pretty uniform.
However in your case, there is a very real difference in the two queries. A project with no Access Level would not appear in the first query, whilst the second query would return with a null AccessName. In effect you would be comparing a LEFT JOIN to an INNER JOIN.
TL:DR For SQL Server/Linq to Entity Framework queries that do the same thing should give similar performance. However your queries are far from similar.
I'm trying to deepen my education about IQueryable custom providers and expression trees. I'm interested in custom parsing a cross-join (viz. SelectMany), and I'm trying to understand if that is exactly what EF is doing when it handles this:
var infoQuery =
from cust in db.Customers
from ord in cust.Orders
where cust.City == "London"
select ord;
Allegedly EF can handle cross joins, though the syntax in that link does not look right to me. Then I found a link with the title "Cross Product Queries" for EF. The syntax looks "correct," but the article itself speaks as if these are normal inner joins rather than cross joins.
Indeed, the code snippet above comes from that last article - and leaves me wondering if EF simply says "I know how these two entities are related, so I will form the inner-join automatically."
What is the real story with EF and this alleged "cross join" sample?
footnote
As I try to build my own IQueryable LINQ provider, the educational goal I've set for myself is to make my own query context for the code snippet above, so that when ToList() is called on the query:
A Console.WriteLine() is automatically fired that prints "This is a cross join of:Customer and Order
The == operator is magically converted into a != before the query is fully interpreted (by an ExpressionVisitor perhaps, not sure).
If someone knows of articles or has snippets of code that would speed my educational goal, please do share! :)
Look closely at the syntax:
from cust in db.Customers
from ord in cust.Orders // cust.
select ...
Because of cust.Orders this is a regular inner join. It's even the preferred way to do a join, because it is far more succinct than the regular join statement.
I don't understand the title of this "Cross-Product Queries" article. Firstly, because as far as I know "cross product" applies to three-dimensional vectors not relational algebra. Secondly, because there is not a single cross join in the examples, only inner joins. Maybe what they're trying to say is that the above syntax looks like a cross join? But it isn't, so it's only confusing to use the word so prominently in the title.
This code snippet
from cust in db.Customers
from ord in db.Orders // db.
select ...
is a true cross join (or Cartesian product). If there are n customers and m orders the result set contains n * m rows. Hardly useful with orders and customers, but it can be useful to get all combinations of elements in two sequences. This construct can also be useful if you want to join but also need a second condition in the join, like
from cust in db.Customers
from ord in db.Orders
where cust.CustomerId == ord.CustomerId && ord.OrderDate > DateTime.Today
Which effectively turns it into an inner join. Maybe not the best example, but there are cases where this comes in handy. It is not supported in the join - on - equals syntax.
I'm trying to bind a LINQ query that has a Distinct() in it and bind it to a Telerik Grid (this is the Telerik grid still, haven't converted to Kendo yet). I've tried adding AsQueryable() to it also, but still getting a "Specified method is not supported exception."
My query looks like this:
var rewardTypes = (from type in _rewardTypeRepository.GetAll()
from vo in type.ValidOutlets
join ot in _outletRepository.GetAll()
on vo.Outlet.Id equals ot.Id
where userOutletIds.Contains(ot.Id)
select vo.RewardType).Distinct().AsQueryable();
Any ideas or guidance on this would be greatly appreciated.
Thank you,
Matt
Using .Distinct() or IQueryable<T> just lazy loads the collection of data, meaning it's not actually enumerated yet.
Try converting it to a List<T>.
var rewardTypes = (from type in _rewardTypeRepository.GetAll()
from vo in type.ValidOutlets
join ot in _outletRepository.GetAll()
on vo.Outlet.Id equals ot.Id
where userOutletIds.Contains(ot.Id)
select vo.RewardType).ToList();
If you want to remain "light" and not enumerate every item in a potentially large collection, I suggest using the .Skip() and .Take() methods to paginate through results at your convenience. Both those methods work against an IQueryable<T> collection.
I am trying to display volunteer information with duty and what performance is allocated.
I want to display this information. However, when I run the query, it did not gather the different date from same performance. And also availability_date is mixed up. Is it right query for it? I am not sure it is right query.
Could you give me some feedback for me?
Thanks.
Query is here.
SELECT Production.name, performance.performance_date, volunteer_duty.availability_date, customer.name "Customer volunteer", volunteer.volunteerid, membership.name "Member volunteer", membership.membershipid
FROM Customer, Membership, Volunteer, volunteer_duty, duty, performance_duty, performance, production
WHERE
Customer.customerId (+) = Membership.customerId AND
Membership.membershipId = Volunteer.membershipId AND
volunteer.volunteerid = volunteer_duty.volunteerid AND
duty.dutyid = volunteer_duty.dutyid AND
volunteer_duty.dutyId = performance_duty.dutyId AND
volunteer_duty.volunteerId = performance_duty.volunteerId AND
performance_duty.performanceId = performance.performanceId AND
Performance.productionId = production.productionId
--Added image--
Result:
The query seems reasonable, in terms of it having what appear to be the appropriate join conditions between all the tables. It's not clear to me what issue you are having with the results; it might help if you explained in more detail and/or showed a relevant subset of the data.
However, since you say there is some issue related to availability_date, my first thought is that you want to have some condition on that column, to ensure that a volunteer is available for a given duty on the date of a given performance. This might mean simply adding volunteer_duty.availability_date = performance.performance_date to the query conditions.
My more general recommendation is to start writing the query from scratch, adding one table at a time, and using ANSI join syntax. This will make it clearer which conditions are related to which joins, and if you add one table at a time hopefully you will see the point at which the results are going wrong.
For instance, I'd probably start with this:
SELECT production.name, performance.performance_date
FROM production
JOIN performance ON production.productionid = performance.productionid
If that gives results that make sense, then I would go on to add a join to performance_duty and run that query. Et cetera.
I suggest that you explicitly write JOINS, instead of using the WHERE-Syntax.
Using INNER JOINs the query you are describing, could look like:
SELECT *
FROM volunteer v
INNER JOIN volunteer_duty vd ON(v.volunteerId = vd.colunteerId)
INNER JOIN performance_duty pd ON(vd.dutyId = pd.dutyId AND vd.volunteerId = pd.colunteerId)
INNER JOIN performance p ON (pd.performanceId = p.performanceId)
I'm trying order a Linq to NHibernate query by the sum of it's
children.
session.Linq<Parent>().OrderBy( p => p.Children.Sum( c => c.SomeNumber ) ).ToList()
This does not seem to work. When looking at NHProf, I can see that it
is ordering by Parent.Id. I figured that maybe it was returning the
results and ordering them outside of SQL, but if I add a .Skip
( 1 ).Take( 1 ) to the Linq query, it still orders by Parent.Id.
I tried doing this with an in memory List and it works just
fine.
Am I doing something wrong, or is this an issue with Linq to
NHibernate?
I'm sure I could always return the list, then do the operations on
them, but that is not an ideal workaround, because I don't want to
return all the records.
To order a query by an aggregate value, you need to use a group by query. In your example you need to use a 'group by' with a join.
The equivelant SQL would be something like:
select id, sum(child.parentid) as childsum from dbo.Parent
inner join child on
parent.id= child.parentid
group by id
order by childsum desc
If only Linq2NH could do this for us... but it can't sadly. You can do it in HQL as long as you're ok with getting the id, and the sum back, but not the whole object.
I battled with Linq2NH for months before I abandoned it. Its slow, doesn't support 2nd level cache and only supports VERY basic queries. (at least when I abandoned it 6 months ago - it may have come along leaps and bounds!) Its fine if you are doing a simple home-made application. If your application is even remotely complex ditch it and spend a bit of time getting to know HQL: a little harder to understand but much more powerful.