I'm working with the criteria API, querying against a mapping file that I cannot really change. There is a root entity with many child entities joined to it and certain queries have required us to add
criteria.SetResultTransformer(CriteriaSpecification.DistinctRootEntity);
in order to avoid getting duplicate entities when there are multiple SQL result-lines due to the joins. The problem is that now I also want to apply an alias-to-bean transformer, like so:
criteria.SetResultTransformer(Transformers.AliasToBean(typeof(MyDto)));
Using either one of these works great. However, I need to combine them: I want to load only the required columns into a DTO object, and also get only distinct root entities. How can I do this?
To load only required column into DTO you can use projection with DistinctEntityRootTransformer as follows
ICriteria criteria = session.CreateCriteria(typeof(YourEntity));
criteria.SetProjection(
Projections.Distinct(Projections.ProjectionList()
.Add(Projections.Alias(Projections.Property("Property"), "Property")));
criteria.SetResultTransformer(
new NHibernate.Transform.AliasToBeanResultTransformer(typeof(MyDto)));
IList list = criteria.List();
I'm a bit green with the criteria api. So far, I've been able to get a list of nhibernate objects by using .List().ToList() on an ICriteria object.
I was just wondering if it was possible, say, to get a list of ints of all the ID #s of the objects that would have been returned, instead of returning the objects.
I know you can do this with HQL but I'm searching the database with dynamic lists from the user gui so the criteria api seems to be the better option.
Thanks.
Isaac
You can use Projections to return only a subset of the properties of an object.
I have a number of database table that looks like this:
EntityId int : 1
Countries1: "1,2,3,4,5"
Countries2: "7,9,10,22"
I would like to have NHibernate load the Country entities identifed as 1,2,3,4,5,7,9 etc. whenever my EntityId is loaded.
The reason for this is that we want to avoid a proliferation of joins, as there are scores of these collections.
Does fetching the countries have to happen as the entity is loaded - it is acceptable that you run a query to fetch these? (You could amend your DAO to run the query after fetching the entity.) The reason I ask is that simply running a query compared to having custom code invoked when entities are loaded requires less "plumbing" and framework innards.
After fecthing your entity, and you have the Country1,Country2 lists, You can run a query like:
select c from Country c where c.id in (:Country1)
passing :Country1 as a named parameter. You culd also retrieve all rows for both sets of countries
select Entity e where e.id in (:Country1, :Country2)
I'm hoping the country1 & country2 strings can be used as they are, but I have a feeling this won't work. If so, you should convert the Strings to a collection of Integers and pass the collection as the query parameter.
EDIT: The "plumbing" to make this more transparent comes in the form of the IInterceptor interface. This allows you to plug in to how entities are loaded, saved, updated, flushed etc. Your entity will look something like this
class MyEntity
{
IList<Country> Country1;
IList<Country> Country2;
// with public getter/setters
String Country1IDs;
String Country2IDs;
// protected getter and setter for NHibernate
}
Although the domain object has two representations of the list - the actual entities, and the list of IDs, this is the same intrusion that you have when declaring a regular ID field in an entity. The collections (country1 and Country2) are not persisted in the mapping file.
With this in place, you provide an IInterceptor implementation that hooks the loading and saving. On loading, you fetch the value of the countryXID property an use to load the list of countries (as I described above.) On saving, you turn the IList of countries into an ID list, and save this value.
I couldn't find the documentation for IInterceptor, but there are many projects on the net using it. The interface is described in this article.
No you cannot, at least not with default functionality.
Considering that there is no SPLIT string function in SQL it would be hard for any ORM to detect the discreet integer values delimited by commas in a varchar column. If you somehow (custom sql func) overcame that obstacle, your best shot would be to use some kind of component/custom user type that would still make a smorgasbond of joins on the 'Country' table to fetch, in the end, a collection of country entities...
...but I'm not sure it can be done, and it would also mean writing from scratch the persistence mechanism as well.
As a side note, I must say that i don't understand the design decision; you denormalized the db and, well, since when joins are bad?
Also, the other given answer will solve your problem without re-designing your database, and without writing a lot of experimental plumbing code. However, it will not answer your question for hydration of the country entities
UPDATE:
on a second thought, you can cheat, at least for the select part.
You could make a VIEW the would split the values and display them as separate rows:
Entity-Country1 View:
EntityId Country
1 1
1 2
1 3
etc
Entity-Country2 View:
EntityId Country
1 7
1 9
1 10
etc
Then you can map the view
Good afternoon, I'm trying to learn nHibernate and it's not terribly clear.
I need to get the result of an sql query:
select patient.name,
discipline.description,
sum(patient.enddate - patient.startdate) as totaltime
from treatment
join patient on patient.id = treatment.patientId
join discipline.id = treatment.disciplineId
I don't need to persist the result, just display it.
If I use hql directly:
What objects will it instantiate and return to me? Will it dynamically build a list of objects containing fields identical to the columns in the result set? The docs leave out this information.
If I need to make a mapping:
Do you create a mapping to a 'meta' object or to one of the joined tables (say 'treatment')?
Thanks
This can be achieved by using DTO objects ("data transfer objects"). You create an object that contains only the data you wish to return. Add an initializing constructor. Then your hql can be of the form:
select new mydto( x.value, y.value ) from x join x.y;
It doesn't seem to like using aggregate functions in the object constructor though.
I'm developing a web- application using NHibernate. Can you tell me how to write a NHibernate Query for the following SQL query:
SELECT v1.Id
FROM VIEW v1
LEFT JOIN VIEW v2 ON v1.SourceView = v2.Id
ORDER BY v1.Position
It's basically a auto-join but I don't know how to write this in Nhibernate. Lets say the property names are the same as the table column names.
You could just perform the select on the original entity and make the association between the two objects "lazy = false". As long as the entities are mapped then both will be returned and you wont get a lazyloadingexception when trying to access the object.
If you don't want to map "lazy=false" then you can also iterate through the results and perform some sort of operation (such as asking if it is null; if(v1.AssocatedObject == null){}) to ensure the data is loaded while the session is open.
Update:
I think there is actually a better one than that in, NHibernateUtil.Initialise() that can initialise a collection without having to wander through it.