Really quick question..
Does Linq2NHibernate always create a left join to retrieve relationships? Is there a way that I can get an inner one instead?
Thank you in advance.
Filipe
As far as I know.
For any relationship that is not One-to-One, NHibernate makes the assumption that the relationship could have 0 to many results, thus uses a Left Join.
I don't know about Linq2NHibernate, but in NH3 using QueryOver API you can specify the join types.
For example, give a Product with a Category. If you wrote:
var result = Session.QueryOver<Product>()
.Fetch(x => x.Category).Eager
.List();
This will result in a left join, if you wanted to specify an inner join you could write the same query as:
var result = Session.QueryOver<Product>()
.JoinQueryOver(x => x.Category, JoinType.InnerJoin)
.List();
This will result in an Inner Join.
As far as I know you can't specify join types when using the LINQ provider (Query<T> in NH3)
Related
I recently used Entity Framework. For elementary CRUD operations, I have no problems, but for more complicated queries, I do not know how to do these.
For example: how to write a nested select? How to use the NOT IN operator?
The query that I want to write is:
SELECT *
FROM course
WHERE idCourse NOT IN (SELECT idCourse
FROM reservation
WHERE idStudent = 'value');
I do not know where to start. Could you please give me some advice?
If I do not misunderstood your question, you want to know how to write the question's query as LINQ-to-Entity query.
Examples could be:
var courses = context.Courses
.Where(c => c.Reservations.All(r => r.idStudent != "value"))
.Select(c => c);
// with eager loading
var courses = (from c in context.Courses.Include(c => c.Reservations)
where c.Reservations.All(r => r.idStudent != "value")
select c).ToArray();
var courses = (from c in context.Courses
join r in context.Reservations on c.idCourse equals r.idCourse
where r => r.idStudent != "value"
select c).ToArray();
The Contains() is equivalent to EXIST IN in a query. Well, 1st and 2nd nearly the same. Only difference is the Include method to show how you could eager load data.
In the 3rd query I use the join key word to do a join operation - equivalent to an INNER JOIN. The result will contain only records where a relation between a course and a reservation exists and the searched student ID is referenced in the reservation.
If you should not use LINQ for your querys, you should take a look. It's a perfect way to decouple your data access layer from persistence layer and you could test all your queries in code too.
Here you could get a very good entry into the subject.
EDIT:
Modified example code to fit NOT IN.
If you are having trouble executing complex queries with LINQ, you can also execute raw SQL queries. They are easy and faster than linq queries.
Here is an example in c#
var list = dc.Database.SqlQuery<YourCourceClass>(#"SELECT *
FROM course
WHERE idCourse NOT IN(SELECT idCourse
FROM reservation
WHERE idStudent = 'value');").ToList();
I have the following SQL I want to create with activerecord. My problem is that I am stuck in a logic loop where I can't LEFT OUTER JOIN a table which has yet to be joined, and I can't find my entry point to the join fiasco
in activerecord I am trying to do
AdMsgs.joins("LEFT OUTER JOIN shows ON ad_msgs.user_id = shows.id OR ad_msgs.user_id = shows.b_id ")
.joins("LEFT OUTER JOIN m ON m.user_id = users.id OR m.m_id = shops.id OR m.m_id = shows.b_id")
.joins("LEFT OUTER JOIN users ON ad_msgs.to = users.email OR ad_msgs.user_id = users.id OR users.id = m.user_id")
.where("shows.id = ?", self.id)
.distinct("ad_msgs.id")
the query outputs an error saying it doesn't know what users is on the second join (probably since I haven't joined it yet) but I need to select the m records according the the users
AdMsgs doesn't have an association with neither of the tables.
Is there a way to full outer join these 3 tables and then select the ones relevant (or any better ways?)
use find_by_sql to implement such scenarios.
otherwise as a rule of thumb, if you can't use joins like this Blog.joins(articles: :comments) you are probably doing something bad or use find_by_sql instead.
in a complex case, i'm writing my query in SQL first to verify the logic involved. often times it's trivial to replace one complex query with 2 simple ones (using IN(*ids)).
I have two Models invoice and payments. The relationship between them is invoice has_many payments.
I'm using the following left outer join to return all invoices that have not been paid at all:
result1 = Invoice.includes(:payments).where(:payments => { :id => nil })
I'm also interested in all invoices that have been partially paid. To return those I use an inner join:
result2 = Invoice.joins(:payments).group("transfers.id").having("sum(payments.amount) < invoice.amount")
I would now like to combine the two results, i.e. I want all Invoices that have either not been paid, or not been fully paid. I know that I could just do result = result1 + result2. However, this doesn't return an ActiveRecord object. Is there a way to combine those two queries in a single query?
I use Rails 4.1 and PostgreSQL
I believe you're correct that you can't get ActiveRecord to generate anything other than an inner join without writing it yourself. But I wouldn't use includes in this context, for while it does happen to cause ActiveRecord to generate a different join, that is an "implementation detail" -- it's fundamental purpose is to load the associated models, which is not necessarily what you want.
In your proposed solution, I don't understand why you'd group by both invoices.id and payments.id -- that seems to defeat the purpose of grouping.
You could do something like the following, although I can't say that this seems much more Rails-ish.
Invoice.joins("LEFT JOIN payments ON payments.transfer_id = invoices.id")
.select("invoices.id, SUM(payments.amount) AS total_paid")
.group("invoices.id")
.having("SUM(payments.amount) IS NULL OR SUM(payments.amount) < invoices.amount")
This will return a list of Invoice objects with only the id field and total_paid field set. If you need other fields available, add them to both the select statement and the the group statement.
I ended up doing just a LEFT OUTER JOIN. However, Rails doesn't seem to support group and having with the LEFT OUTER JOIN generated by includes(). Therefore, i had to construct the join manually which actually doesn't feel like "the Rails way".
My query:
Invoice.joins("LEFT JOIN payments ON payments.transfer_id = invoices.id").group("invoices.id, payments.id").having("((payments.id IS NULL) OR (sum(payments.amount) < invoices.amount))")
Update: This doesn't work as expected. Please see the accepted answer.
My apologies for my recent SQL/Linq questions, but i need to know what this working SQL query would look like in LINQ?
select *
from CarePlan c
outer apply (select top 1 * from Referral r
where
r.CarePlanId = c.CarePlanId order by r.ReferralDate desc) x
left outer join Specialist s on s.SpecialistId = x.SpecialistId
left outer join [User] u on u.UserId = s.UserId
This basically retrieves a list of Careplans with the newest Referral (if it exists), then joins the Specialist and User table based on any found Referrals
Thanks
Kind advice: target on what you want to express in the environment of your class model and LINQ, in stead of trying to reproduce SQL. If you do something like
context.CarePlans
.Select(cp => new { Plan = cp, FirstReferral = cp.Referrals.FirstOrDefault() }
(provided that it matches your context and ignoring ordering and other joins for clarity)
It would basically do what you want, but it may very well translate to an inline subquery, rather than an OUTER APPLY. To the same effect. And the execution plan probably won't differ very much.
I have a reference to call that may or may not be there. When I add the nullable option it still doing Inner Join when I want an Left Join (show the left even if the right is null). Is this possible in the NH map?
References(x => x.DefaultCategory, "CATEGORY_ID")
.Nullable();
The join type isn't specified in the mapping, but in the query.
IN HQL, use left join syntax as with SQL.
With Criteria, use:
.CreateAlias("categories", "c", JoinType.LeftOuterJoin)