I have a requirement to use a reporting-friendly query from HQL. That is, each column is represented by a standard Java type (Long, Integer, String, List) rather than a mapped class. The values will be used by a third party library, so I have very limited control over post-processing.
My example object tree looks like this:
a.x
a.y (a collection of z)
a.y.z[0].v
a.y.z[1].v
a.y.z[2].v
I would like query to retrieve two columns. The first column is the plain "a.x" field, and the second is a String of a comma separated list of all of the a.y.z.v values. If this is not possible, then having the second column return as a Java list of the a.y.z.v values would be satisfactory.
In short, I would like to flatten the a.y.z.v collection to a csv String or to a List object from inside the query.
I have already attempted the following:
Using the "new" keyword in a list subselect. ie, "select a.x, (select new list(a.y.z.v)) from a". If necessary I could have transformed the contents of the list into a csv, but this caused a syntax error.
Using the "new" keyword with a custom object in a subselect. ie, "select a.x, (select new custom.package.ListToCsvObject(a.y.z)) from a". This caused the same error as the first attempt
Using the "elements()" keyword in the select. Unfortunately, this keyword only seems to work inside "in", "exists" clauses (etc), not as the actual returned value.
The only solution we've been able to find was to create a stored procedure in the database and use that, but such a solution is painfully slow through HQL (it turns a sub-second query into a 30 second query) and therefore is not something we want to continue to do.
I am able to make some limited changes to the Hibernate mapping (so I can add #formula, etc) but I would prefer not to have to make major changes to the database schema to support it. (So no, I don't want to create a denormalized "csv_value" column in the database!)
Could anyone suggest some code or, failing that, an alternative approach to solving this problem?
Try something like this should work. Flattening of list to comma separated string is done in the constructor of your VO class. You can also take a look at resultTransformer, you can create a custom resultTransformer and attach it to the query.
class ResultVO {
String x;
String y;
public ResultVO(String x,List<Z> y) {
this.x = x;
this.y = createCSV(y);
}
}
then in HQL
select new ResultVO(a.x,a.y) from a
A warning - this is not a good way to use JPA. If most of your use cases are like this you should seriously reconsider using some other persistence approach (ibastis, spring jdbc template + sql etc).
Related
I have a situation where I need to return only few fields of a POJO.
Here is a SOF Question: retrieve-single-field-rather-than-whole-pojo-in-hibernate question regarding the same, but few things still seems to be obscure.
1) The answer suggests to use -
String employeeName = session.createQuery("select empMaster.name from EmployeeMaster empMaster where empMaster.id = :id").setInteger("id",10).uniqueResult();
So, here is my concern - Every pojo field is normally private, so "empMaster.name" will simply not work. And am not sure if empMaster.getName() is the solution for this. Will calling the getter methods work?
2) If i am querying multiple fields, (which is my case) (assuming getter methods work) the query will be some thing like -
List<String> employeeDetails = session.createQuery("select empMaster.getName(), empMaster.getDesignation() from EmployeeMaster empMaster where empMaster.id = :id").setInteger("id",10).uniqueResult();
Note the return type has changed from String to List<String>.
2(a) Hope this is right?
2(b) what if i am interested in age/salary of employee which will be of int type. I think the return type will be List<String> or List<Object>. Well any how in the application i can recast the String or Object to the proper type int or float etc. So this should not be a problem.
3) Now what if I am querying multiple employee details (no where clause), so the query will be something like - (not sure if the part of query after from is correct)
List<List<<String>> employeesDetails = session.createQuery("select empMaster.getName(), empMaster.getDesignation() from EmployeeMaster;
Anyway, point here is to emphasise the change in the return type to : List<List<<String>> employeesDetails. Does it work this way ???.
(The question quoted above also has answers pointing to use Projections. I have questions about it but will post them on another question, don't want to mixup.)
I will list the points in the order you mentioned them:
The query has nothing to do with the POJO's field visibility. You are doing a simple query to the database, as if you were doing a query using SQL, and columns in a table have nothing to do with the fact that their mapped POJOs' fields in an application are public or private. The difference is only the language that you're using: now you're using the Hibernate Query Language (HQL), which allows you to express your query with respect to the POJOs' definitions instead of the database's tables' definitions. In fact, doing
session.createQuery("select empMaster.getName() from EmployeeMaster...");
will throw a syntax error: there can be no parenthesis in an object's field name.
By the way, you have to parse your query result to a String, otherwise there would be a compiler semantics error.
String name = (String) session.createQuery("select empMaster.name from EmployeeMaster...").setYourParameters().uniqueResult();
When you do a query whose SELECT clause contains more than one field, and you call uniqueResult(), you're obtaining an array of Objects (Object[]). You iterate through each element in the array and cast it to the corresponding type.
First of all, you probably forgot to add a .list() method call at the end of your query.
session.createQuery("select empMaster.name, empMaster.designation from EmployeeMaster").list();
Otherwise, the query will not be executed, and you would just have a QueryImpl object waiting to be triggered.
Now, when you're expecting multiple results in your query and selecting several fields, you're getting back a List<Object[]>. The elements of the list will be the array of fields for a specific record, and the array per se will be the fields.
While building a query using Hibernate, I noticed something rather odd. If I use sequential named parameters for the ORDER BY clause, Hibernate throws a QuerySyntaxException (the colon prefix being an unexpected token):
createQuery("FROM MyEntity ORDER BY :orderProperty :orderDirection");
However, when this is done with a plain SQL query the query is created without a problem:
createSQLQuery("SELECT * FROM my_entity_table ORDER BY :orderProperty :orderDirection");
I know Hibernate is doing more String evaluation for the HQL query, which is probably why the SQL query is created without an error. I am just wondering why Hibernate would care that there are two sequential named parameters.
This isn't a huge issue since it is simple to work around (can just append the asc or desc String value to the HQL instead of using a named paramater for it), but it struck my curiosity why Hibernate is preventing it (perhaps simply because 99% of the time sequential named parameters like this result in invalid SQL/HQL).
I've been testing this in my local, and I can't get your desired outcome to work with HQL.
Here is quote from the post I linked:
You can't bind a column name as a parameter. Only a column value. This name has to be known when the execution plan is computed, before binding parameter values and executing the query. If you really want to have such a dynamic query, use the Criteria API, or some other way of dynamically creating a query.
Criteria API looks to be the more useful tool for your purposes.
Here is an example:
Criteria criteria = session.createCriteria(MyEntity.class);
if (orderDirection.equals("desc")) {
criteria.addOrder(Order.desc(orderProperty));
}
else {
criteria.addOrder(Order.asc(orderProperty));
}
According to the answer accepted in this question, you can only define parameters in WHERE and HAVING clauses.
The same answer also gives you some ways to have a workaround for your problem, however I will add one more way to do this:
Use the CASE - WHEN clause in your ORDER BY, this would work by the following way:
SELECT u FROM User u
ORDER BY
CASE WHEN '**someinputhere**' = :orderProperty
AND '**someotherinput**' = :orderDirection
THEN yourColumn asc
ELSE yourColumn desc END
Please, note that in this approach would required you to write all the possible inputs for ordering. Not really beautiful but really useful, especially because you would not need to write multiple queries with different orderings, plus with this approach you can use NamedQueries, which would be possible by writing the query dinamically using string concats.
Hope this can solve your problem, good luck!
I would like to get objects via Hibernate from database with concrete order. This order is something like that:
as the first I would like to get objects with column titled for example first_column not null,
as the second I would like to get objects with column second_column not null,
as the last I would like to get objects which third_column is the id for another object/table, and this another object has a field with concrete value for example: "something".
I have created criteria in this way:
criteria.addOrder(Order.asc("firstColumn"));
criteria.addOrder(Order.asc("secondColumn"));
but how can I meet the last requirement?
With the restriction I can do something like that:
criteria.createAlias("thirdColumn", "t");
criteria.add(Restrictions.eq("t.field", "something"));
But I have to use order, not restriction with three separate Criteria results, because I am using also setFirstResult() and setMaxResults() of the Criteria to implement pagination in my frontend.
If you can write the statement in SQL then you can probably get away with the approach mentioned in this post which is to create a custom subclass of Order.
I think you can simply use the "." separator and write your code as follow
criteria.createAlias("thirdColumn", "t");
criteria.addOrder(Order.asc("t.field"));
I'm trying to figure out how to select a list of objects as part of a Hibernate group-by query. I know how to do it a harder way, but I'm curious if there is some special sugar syntax that achieves the same thing.
Basically, I have a query of this structure:
select com.myapp.domain.TagSummary(
tag.id, tag.term, tag.description, tag.synonyms, count(user)
)
from User user
join user.tags tag
I'd like to store the tag.synonyms as a List<Tag>. Is that possible, or do I need to query the cross product and do the separation manually after the query results come back?
Alternatively, what I really want in the end is a list of synonym terms separated by commas. So if a tag is spring and it has synonym terms spring-framework and spring-framework-3.1, it would be great to put into the constructor the string spring-framework, spring-framework-3.1. Is that possible?
EDIT: I have learned that I can use group_concat() to achieve the second half of the functionality, but it's only available in MySQL. Is there a way to make it available in hsqldb as well? In Spring 3.1, how do I add this function to Hibernate? I know I should call something on Configuration, but I don't know what bean to access it by.
for (Object[]> result : query.list()) {
Tag tag = (Tag ) result[3];
User user = (User) result[4];
}
You can get more information from this link
https://derrickpetzold.com/p/in-and-group-by-count-hibernate/
Let's depict the following use case: I have a JPQL Query which on the fly creates data objects using the new keyword. In the SELECT clause I would like to inject an attribute which is not known to the database but to the layer which queries it.
This could look like
EntityManager em; // Got it from somewhere
boolean editable = false; // Value might change, e.g. depending on current date
Query q = em.createQuery("SELECT new foo.bar.MyDTO(o, :editable) FROM MyObject o")
.setParameter("editable", editable);
List<MyDTO> results = (List<MyDTO>) q.getResultList();
Any ideas how this kind of attribute or parameter injection into the SELECT clause might work in JPQL? Both JPA and JPA 2.0 solutions are applicable.
Edit: Performance does not play a key role, but clarity and cleanness of code.
Have you measured a performance problem when simply iterating over the list of results and call a setter on each of the elements. I would guess that compared to
the time it takes to execute the query over the database (inter-process call, network communication)
the time it takes to transform each row into a MyObject instance using reflection
the time it takes to transform each MyObject instance into a MyDTO using reflection
your loop will be very fast.
If you're so concerned about performance, you should construct your MyDTO instances manually from the returned MyObject instances instead of relying on Hibernate and reflection to do it.
Keep is simple, safe, readable and maintainable first. Then, if you have a performance problem, measure to detect where it comes from. Then and only then, optimize.
It will not work without possible vendor extensions, because according specification:
4.6.4 Input Parameters
...
Input parameters can only be used in the
WHERE clause or HAVING clause of a query.