i have a complex entity, very very heavy.
i want to select only the id and the name of this entity from the db for a better performance.
how do i do that with nhibernate (or fluent nhibernate).
There are a few different possibilities.
Create a New Entity
One possible solution is to create a new entity and map it to the same table, but only map the columns that you want (id and name). This is quite flexible and lets you use that entity as any other entity. The problem is that you introduce some duplication.
Using HQL
Another solution is to use projections. With HQL you can use the select clause to specify which columns to retrieve. If you want a proper entity instance as the result from the query and not an array of objects, you can create a custom constructor for your class and use that in the HQL query.
session.CreateQuery("select new Foo(f.Id, f.Name) from Foo f").List<Foo>();
Using the Criteria API
If you want to use the Criteria API instead of HQL, you can use the SetProjection method. If you want a proper entity from the query and not an array of objects, you can use the AliasToBean result transformer.
session.CreateCriteria(typeof(Foo))
.SetProjection(Projections.ProjectionList()
.Add(Projections.Property("Name"), "Name")
.Add(Projections.Property("Id"), "Id"))
.SetResultTransformer(Transformers.AliasToBean(typeof(Foo)))
.List();
The criteria example code is borrowed from the following question, which might be of interest:
NHibernate - Only retrieve specific columns when using Critera queries?
You have 2 choices :
when using ICriteria - use (syntax might not be correct but you get the picture)
SetProjections(Projections.ProjectionList.Add(Projections.Property("prop"))
.Add(Projections.Property("prop1")))
when using hql
select c.Col1,c.Col2 from table c
Both of them will return a list of array lists - or something like that - you then have to traverse it and build your .. dictionary (or whatever you have).
Related
In our webapp we have a number of places where you would be updating a number of tables in one complex form/view. In raw SQL I would probably select a bunch of columns from a bunch of tables and edit that one record on the primary table as well as related parent/child tables.
In hibernate I would probably just pull a JPA entity for the main table and let hibernate fetch the parent/child relationships as I populate the view. And then later pull from my view back to the entity and call entitymanger .perist/merge.
In JOOQ I have a number of options but it appears you can pull a main record via selectFrom/fetch then use fetchChild fetchParent to pull typed related records like so...
LoadsRecord load = dslContext.selectFrom(LOADS)
.where(LOADS.ID.eq(id))
.fetchOne();
SafetyInspectionsRecord safetyInspection = load.fetchParent(Keys.LOADS__FK_SAFETY_INSPECTION);
So this way I am able to pull related records in a typesafe manner. The only annoying thing is I have to run another full query every time I call fetchParent or fetchDhild. Is there a way to eagerly fetch these all at once to avoid multiple round trips to the DB?
It is really nice to have these classes like LoadsRecord for CRUD screens, it makes updating the DB easy.
Classic approach using joins
There are various ways you can achieve materialising a to-one relationship. The simplest one being a simple JOIN or LEFT JOIN if the relationship is optional.
E.g.:
Result<?> result =
ctx.select()
.from(LOADS)
.join(SAFETY_INSPECTIONS)
.on(LOADS.SAFETY_INSPECTIONS_ID.eq(SAFETY_INSPECTIONS.ID))
.fetch();
You probably want to work with the generated records thereafter, so you can use various mapping tools to map the generic Record types to the two UpdatableRecord types for further CRUD:
for (Record r : result) {
LoadsRecord loads = r.into(LOADS);
SafetyInspectionsRecord si = r.into(SAFETY_INSPECTIONS);
}
Using nested records
Starting from jOOQ 3.15 and #11812, MULTISET and ROW operators can be used to create nested collections and records. So, in your query, you could write:
Result<?> result =
ctx.select(
row(LOADS.ID, ...),
row(SAFETY_INSPECTIONS.ID, ...))
.from(LOADS)
.join(SAFETY_INSPECTIONS)
.on(LOADS.SAFETY_INSPECTIONS_ID.eq(SAFETY_INSPECTIONS.ID))
.fetch();
That would already help map the nested data structures into the desired format. Starting from jOOQ 3.17 and #4727, you can even use table expressions directly to generate nested records:
Result<Record2<LoadsRecord, SafetyInspectionsRecord>> result =
ctx.select(LOADS, SAFETY_INSPECTIONS)
.from(LOADS)
.join(SAFETY_INSPECTIONS)
.on(LOADS.SAFETY_INSPECTIONS_ID.eq(SAFETY_INSPECTIONS.ID))
.fetch();
This new feature is definitely going to close one of jOOQ's biggest gaps. You could even simplify the above using implicit joins to this:
Result<Record2<LoadsRecord, SafetyInspectionsRecord>> result =
ctx.select(LOADS, LOADS.safetyInspections())
.from(LOADS)
.fetch();
I have a named hql query which makes use of object constructors for an object that is not mapped (it is only imported)
e.g.
select distinct new NotMappedResultClass(ah.SomeProp1, ah.SomeProp2)
from SomeMappedClass
where ...
order by ah.SomeProp1
The weird thing is, that when I call IQuery.List() in NHibernate, I end up with exactly twice as many rows from NHibernate than from the query that NHibernate ran (traced using SqlProfiler).
(In case it matters, the "where" clause does actually involve some subqueries).
Why is NHibernate duplicating the rows coming back from the database?
(I am using NHibernate 1.2.1.4000)
Found the problem.
My project has some odd mappings when it comes to inherited classes.
Basically, SomeMappedClass was abstract, and had its own NHibernate mapping, and there was a derived class SomeDerivedClass (that didn't add any functionality) that was also mapped separately without the "extends" attribute.
This caused NHibernate to issue two sql queries, with different aliases for the same table.
In my case the simple quick and dirty solution was to query from SomeDerivedClass instead of SomeMappedClass, but the more appropriate solution would probably be to modify the mappings / object inheritance.
We are using Oracle 10g database, NHibernate, WCF and Silverlight 3.0 in our project
The situation we have is that the entities in my project have many properties. But for certain situations, like showing the options in dropdown, I only want to retrieve the list of ID and Name field for that entity. I do not want to return a list of the entire entity object as a whole as there are many columns in the table. Presently I am using two SELECT queries: one to fetch the list of IDs and second to fetch the list of Names separately. Then I join these two queries and form a Dictionary and pass it to the UI.
The concern for me is that would it be possible to achieve this in a single query itself?
One approach that I know of is to create a new class having only the ID and Name property, import it into NHiberante and then form a list of this new class and send it to the UI. I want to avoid this approach for now as there are many tables for which I have to implement this functionality and hence I will have to create many new classes and corresponding xml files.
Any sort of help would be greatly appreciated.
Here is one way to do it using the Criteria API, Projections and AliasToBean. If it's a simple non-persistent class containing Id and Name you can reuse the class. NHibernate query CreateCriteria
Some quick nhibernate problem:
I have sql tables:
Item { Id, Name }
ItemRange { Id, Name }
ItemHasItemRange { Id, ItemId, ItemRangeId }
Mappings are simple, so I will not paste them, the ItemId and ItemRangeId are foreign keys, Item class has ItemHasItemRanges collection mapped as lazy bag.
I want all items which are in particular ItemRange, but I do not want to retrieve associated ItemRangeObjects, I just want to do inner join to narrow results.
When I do it like that:
c.CreateCriteria("Item", "i")
.CreateAlias("ItemHasItemRanges", "ihpr", JoinType.InnerJoin)
.Add(Restrictions.Eq("ihpr.ItemRange.Id", I18nHelper.CurrentItemRange.Id));
It works fine, but all ItemHasItemRange objects are fetched as well to the Item.ItemHasItemRanges collections (which is mapped as lazy)
I do not want to fetch Item.ItemHasItemRanges, because it takes time. I just want to do inner join to limit result set. It is possible in NHibernate?
So I think that you just want to retrieve those objects in order to show an overview / list, and you are not going to actually 'do' something with those objects (unless perhaps loading one of them) ?
In that case, I think that it is better for you to work with 'projections'.
Here's the scenario:
You'll have to create a (simple) class that just contains the properties that you want to show (where you're interested in).
You'll have to 'import' that class into NHibernate, so that NHibernate knows of its existence.
Next, you can create your Criteria statement like you have it now. (Working with your domain classes).
Then, you should specify how the projection should look like. That is, how the properties of your Item entity map to the properties of your 'DTO'/View class (= the simple class you just created).
Specify that an AliasToBean ResultTransformer should be used.
Then, execute your Criteria query. NHibernate will be able to produce the simplest possible query that is needed in order to retrieve all the data that is necessary.
I've explained something similar here
I find out the problem was somewhere else. ItemHasItemRange table did not have multiple index on ItemId and ItemRangeId - id only had separate indexes on each field. Thats why performance was so poor.
But NHibernate question is still valid - is it possible to create inner join for criteria only to narrow results and not to fetch all joined objects which normally are lazy.
I need to restrict a one-to-many collection by a date column with a value specified on the parent element. The 'where' attribute on set or bag looks perfect for this.
NHibernate documentation describes the 'where' attribute as being for arbitrary SQL, so am I correct in assuming that I cannot use values from the parent class here as I would in HQL and must implement my own IUserCollection instead?
IMHO using a filter would be better because you could write the restriction in database agnostic HQL.