How can HQL be used to select specific objects that meet a certain criteria?
We've tried the following to generate a list of top ten subscribed RSS feeds (where SubscriptionCount is a derived property):
var topTen = UoW.Session.CreateQuery( #"SELECT distinct rss
FROM RssFeedSubscription rss
group by rss.FeedUrl
order by rss.SubscriptionCount DESC
")
.SetMaxResults(10)
.List<RssFeedSubscription>();
Where the intention is only to select the two unique feed URLs in the database, rather than the ten rows int the database instantiated as objects. The result of the above is:
Column 'RssSubscriptions.Id' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
ORDER BY items must appear in the select list if SELECT DISTINCT is specified.
It's possible just to thin out the results so that we take out the two unique feed URLs after we get the data back from the database, but there must be a way to do this at the DB level using HQL?
EDIT: We realise it's possible to do a Scalar query and then manually pull out values, but is there not a way of simply specifying a match criteria for objects pulled back?
If you change your HQL a bit to look like that:
var topTen = UoW.Session.CreateQuery( #"SELECT distinct rss.FeedUrl
FROM RssFeedSubscription rss
group by rss.FeedUrl
order by rss.SubscriptionCount DESC
")
.SetMaxResults(10)
.List();
the topTen variable will be an object[] with 2 elements in there being the 2 feed URLs.
You can have this returned as strongly typed collection if you use the SetResultTransformer() method of the IQuery interfase.
You need to perform a scalar query. Here is an example from the NHibernate docs:
IEnumerable results = sess.Enumerable(
"select cat.Color, min(cat.Birthdate), count(cat) from Cat cat " +
"group by cat.Color"
);
foreach ( object[] row in results )
{
Color type = (Color) row[0];
DateTime oldest = (DateTime) row[1];
int count = (int) row[2];
.....
}
It's the group by rss.FeedUrl that's causing you the problem. It doesn't look like you need it since you're selecting the entities themselves. Remove that and I think you'll be good.
EDIT - My apologies I didn't notice the part about the "derived property". By that I assume you mean it's not a Hibernate-mapped property and, thus doesn't actually have a column in the table? That would explain the second error message you received in your query. You may need to remove the "order by" clause as well and do your sorting in Java if that's the case.
Related
I recently started using GraphQL through a Hasura layer on top of a PostgreSQL DB. I am having troubles implementing a basic query.
Here are my entities :
article :
articleId
content
publishedDate
categoryId
category :
categoryId
name
createdDate
What I am trying to achieve, in English, is the following : get the articles that were published in the first 3 days following the creation of their related category.
In pseudo-SQL, I would do something similar to this :
SELECT Article.content, Category.name
FROM Article
INNER JOIN Category ON Article.categoryId = Category.categoryId
WHERE Article.publishedDate < Category.createdDate + 3 days
Now as a GraphQL query, I tried something similar to this :
query MyQuery {
articles(where: {publishedDate: {_lt: category.createdDate + 3 days}}) {
content
category {
name
}
}
}
Unfortunately, it does not recognise the “category.createdDate” in the where clause. I tried multiple variations, including aliases, with no success.
What am I missing ?
To my understanding of the Hasura docs, there is no way to reference a field within a query like you can do in SQL. But that does not mean, you can't do, what you are trying to do. There are three ways of achieving the result that you want:
1. Create a filtered view
CREATE VIEW earliest_articles AS
SELECT Article.*
FROM Article
INNER JOIN Category ON Article.categoryId = Category.categoryId
WHERE Article.publishedDate < Category.createdDate + 3 days
This view should now become available as a query. Docs for views are here.
2. Create a view with a new field
CREATE VIEW articles_with_creation_span AS
SELECT
Article.*,
(Article.publishedDate - Category.createdDate) AS since_category_creation
FROM Article
INNER JOIN Category ON Article.categoryId = Category.categoryId
Now you can again query this view and use a filter query on the extra field. This solution is useful if you want to vary the amount of time, that you want to query for. The downside is, that there are now two different article types, it might make sense to hide the regular articles query then.
3. Use a computed field
You can also define computed fields in Hasura. These fields are not only included in the output object type of the corresponding GraphQL type, but they can also be used for querying. Refer to the docs on how to create a computed field. Here you can again, calculate the difference and then use some kind of comparison operator (<) to check if this time is smaller than '3 days'.
I have this problem, I've got database table that looks like this:
"63";"CLINICAL...";"Please...";Blah...;"2014-09-23 13:15:59";37;8
"64";"CLINICAL...";"Please...";Blah...;"2014-09-23 13:22:51";37;9
The values that matter are the second to last and last one.
As you can see, the second to last (abstract_category_numbers) are the same, but the last differs (version_numbers)
Here is the problem:
When I make a scope, it returns all of the records, which i need to focus on the one with the maximum version number.
In SQL i would do something like this:
'SELECT * FROM Category c WHERE
NOT EXISTS SELECT * FROM Category c1
WHERE c.version_number < c1.version_number
AND c.abstract_category_id = c1.abstract_category_id'
But i'm totally lost at Ruby, more specifically how to do this kind of select in the scope (I understand it should be a relation)
Thanks
We can create a scope to select the category with max version_number like this:
scope :with_max_version_number, -> {
joins("JOIN ( SELECT abstract_category_id, max(version_number) AS max_version
FROM categories
GROUP BY abstract_category_id
) AS temp
ON temp.abstract_category_id = categories.abstract_category_id
AND temp.max_version = categories.version_number"
)
}
Basically, we will select the category with the max_version value on temp table in the subquery.
Btw, I expect the table name is categories, you may correct it. Then the final query will be:
Category.with_max_version_number
Scopes are suppose to return an array of values even if there is only 1 record.
If you want to ALWAYS return 1 value, use a static method instead.
def self.max_abstract_category
<your_scope>.max_by{ |obj| obj.version_number }
end
If I understand your question: you have a database table with a version_number column, which rails represents using an Active Record model--that I'll call Category because I don't know what you've called it--and you want to find the single Category record with the largest version_number?
Category.all.order(version_numbers: :DESC).limit(1).first
This query asks for all Category records ordered by version_number from highest to lowest and limits the request to one record (the first record, a.k.a the highest). Because the result of this request is an array containing one record, we call .first on the request to simply return the record.
As far as I'm aware, a scope is simply a named query (I don't actually use scopes). I think you can save this query as a scope by adding the following to your Category model. This rails guide explains more about Scopes.
scope :highest_version, -> { all.order(version_numbers: :DESC).limit(1).first }
I join implementation with baby_squeel but for some reason it was very slow on mysql. So I ended up with something like:
scope :only_latest, -> do
where(%{
NOT EXISTS (SELECT * FROM Category c
WHERE categories.version_number < version_number
AND categories.abstract_category_id = abstract_category_id')
}
end
I filed a BabySqueel bug as I spent a long time trying to do in a code proper way to no avail.
I am using SQL server 2005, querying with Web Developer 2010, and the min function appears to be returning more than one value (for each ID returned, see below). Ideally I would like it to just return the one for each ID.
SELECT Production.WorksOrderOperations.WorksOrderNumber,
MIN(Production.WorksOrderOperations.OperationNumber) AS Expr1,
Production.Resources.ResourceCode,
Production.Resources.ResourceDescription,
Production.WorksOrderExcel_ExcelExport_View.PartNumber,
Production.WorksOrderOperations.PlannedQuantity,
Production.WorksOrderOperations.PlannedSetTime,
Production.WorksOrderOperations.PlannedRunTime
FROM Production.WorksOrderOperations
INNER JOIN Production.Resources
ON Production.WorksOrderOperations.ResourceID = Production.Resources.ResourceID
INNER JOIN Production.WorksOrderExcel_ExcelExport_View
ON Production.WorksOrderOperations.WorksOrderNumber = Production.WorksOrderExcel_ExcelExport_View.WorksOrderNumber
WHERE Production.WorksOrderOperations.WorksOrderNumber IN
( SELECT WorksOrderNumber
FROM Production.WorksOrderExcel_ExcelExport_View AS WorksOrderExcel_ExcelExport_View_1
WHERE (WorksOrderSuffixStatus = 'Proposed'))
AND Production.Resources.ResourceCode IN ('1303', '1604')
GROUP BY Production.WorksOrderOperations.WorksOrderNumber,
Production.Resources.ResourceCode,
Production.Resources.ResourceDescription,
Production.WorksOrderExcel_ExcelExport_View.PartNumber,
Production.WorksOrderOperations.PlannedQuantity,
Production.WorksOrderOperations.PlannedSetTime,
Production.WorksOrderOperations.PlannedRunTime
If you can get your head around it, I am selecting certain columns from multiple tables where the WorksOrderNumber is also contained within a subquery, and numerous other conditions.
Result set looks a little like this, have blurred out irrelevant data.
http://i.stack.imgur.com/5UFIp.png (Wouldn't let me embed image).
The highlighted rows are NOT supposed to be there, I cannot explicitly filter them out, as this result set will be updated daily and it is likely to happen with a different record.
I have tried casting and converting the OperationNumber to numerous other data types, varchar type returns '100' instead of the '30'. Also tried searching search engines, no one seems to have the same problem.
I did not structure the tables (they're horribly normalised), and it is not possible to restructure them.
Any ideas appreciated, many thanks.
The MIN function returns the minimum within the group.
If you want the minimum for each ID you need to get group on just ID.
I assume that by "ID" you are referring to Production.WorksOrderOperations.WorksOrderNumber.
You can add this as a "table" in your SQL:
(SELECT Production.WorksOrderOperations.WorksOrderNumber,
MIN(Production.WorksOrderOperations.OperationNumber)
FROM Production.WorksOrderOperations
GROUP BY Production.WorksOrderOperations.WorksOrderNumber)
Hey - I'm battling to figure out how to write the following using NHibernate ICriteria (Multi criteria?) for the following:
(It's a query to get a list of first names ordered by popularity in the table in the last day)
select firstname,count(firstname) as occurances from registrants
where timestamp between DateAdd(day,-1, GetDate()) and getdate()
group by firstname
order by count(firstname) desc
Also, given this is just a couple of columns from a table, excluding the ID, and NHibernate needs ID's for it's objects, what's the easiest way to "fake" an ID so I can just get the results?
You need to use projections and a transformer to do this. Here's some background info http://nhibernate.info/doc/nh/en/index.html#querycriteria-projection
var criteria = Session.CreateCriteria<Registrant>()
.Add(Restrictions.Between("Timestamp", DateTime.Now.AddDays(-1), DateTime.Now)
.AddOrder(Order.Desc(Projections.Count("FirstName")))
.SetProjection(Projections.ProjectionList()
.Add(Projections.GroupProperty("FirstName"), "FirstName")
.Add(Projections.Count("FirstName"), "Occurances")
.SetResultTransformer(Transformers.AliasToBean<FirstNameOccurance>());
criteria.List<FirstNameOccurance>();
You'll need to create a class called FirstNameOccurance that has 2 properties called FirstName and Occurances.
How to get value of some fields in a native query (JPA)?
For example I want to get name and age of customer table:
Query q = em.createNativeQuery("SELECT name,age FROM customer WHERE id=...");
Note: I don't want to map results to entities. I just want to get the value of the field.
Thanks
A native query with multiple select expressions returns an Object[] (or List<Object[]>). From the specification:
3.6.1 Query Interface
...
The elements of the result of a Java
Persistence query whose SELECT clause
consists of more than one select
expression are of type Object[]. If
the SELECT clause consists of only one
select expression, the elements of the
query result are of type Object. When
native SQL queries are used, the SQL
result set mapping (see section
3.6.6), determines how many items (entities, scalar values, etc.) are
returned. If multiple items are
returned, the elements of the query
result are of type Object[]. If only a
single item is returned as a result of
the SQL result set mapping or if a
result class is specified, the
elements of the query result are of
type Object.
So, to get the name and age in your example, you'd have to do something like this:
Query q = em.createNativeQuery("SELECT name,age FROM customer WHERE id = ?1");
q.setParameter(1, customerId);
Object[] result = (Object[])q.getSingleResult();
String name = result[0];
int age = result[1];
References
JPA 1.0 specification
Section 3.6.1 "Query Interface"
Section 3.6.6 "SQL Queries"
Depends on the JPA implementation. Hibernate does it different than castor, for example.
Here's a example how it would work in Hibernate:
Query query = session.createSQLQuery(
"select s.stock_code from stock s where s.stock_code = :stockCode")
.setParameter("stockCode", "7277");
List result = query.list();
I don't think that it is possible to get a plane value such as an integer... but this one should come very close to it.