I have a nHibernate query like this
ICriteria query = session.CreateCriteria(typeof(MyResult))
.Add(Expression.Eq("ResultTypeId", myResult.ResultTypeId))
Problem is that users add results all the time and I want to show a table of all the latest results for all the diferent ResultTypes I have.
The MyResult class has a property ResultDate. My question is, what do I add to the query to get it to only return the latest result for the given result type. There is nothing to say that the results will be in date order in the database.
Thanks,
Mark
You can order the result by ResultDate using the AddOrder method, as below:
ICriteria query = session.CreateCriteria(typeof(MyResult))
.Add(Expression.Eq("ResultTypeId", myResult.ResultTypeId))
.AddOrder(Order.Desc("ResultDate"))
.List<MyResult>();
If you want to limit the number of MyResult instances you get back, you can use the SetMaxResults method, like so:
ICriteria query = session.CreateCriteria(typeof(MyResult))
.Add(Expression.Eq("ResultTypeId", myResult.ResultTypeId))
.AddOrder(Order.Desc("ResultDate"))
.SetMaxResults(20)
.List<MyResult>();
If I understand the question well, Mark wants to see an overview of all the last results for each type.
Which means that, for every result type, he only wants to see only one row, and that is the Result which has last been added for that type.
I think that, the easiest way to achieve this, would be to create an additional class, which we can call 'MyResultOverview' for instance:
public class MyResultOverview
{
public int ResultId {get; set;}
public int ResultTypeId {get; set;}
public DateTime ResultDate {get; set;}
}
This class should not be mapped, but NHibernate should be aware that this class exists. Therefore, we'll have to import it:
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" >
<import class="MyResultOverview" />
</hibernate-mapping>
Then, we can create an ICriteria which will populate instances of MyResultOverview (and which will also generate the most efficient SQL Query in order to get this overview).
It should look something like this:
ICriteria criteria = session.CreateCritera (typeof(MyResult));
criteria.SetProjection (Projections.ProjectionList ()
.Add (Projections.Property("Id"), "ResultId")
.Add (Projections.Property("ResultType"), "ResultType")
.Add (Projections.Max("ResultDate"), "ResultDate"));
criteria.SetResultTransformer (Transformers.AliasToBean (typeof(MyResultOverview)));
IList<MyResultOverview> results = criteria.List<MyResultOverview>();
This should give you a list of MyResultOverview instances which represent the MyResults that you're looking for.
Then, in order to retrieve the MyResult itself, you can simply do this by retrieving the MyResult instance for that particalur ResultId that you've retrieved as well.
I haven't tested this, nor did i compile it, but this is the path that I would follow to achieve this.
Order by ResultDate (descending) and select top whatever you feel appropriate.
In HQLthis might work:
select item, tag
from MyItem item
join item.Tags tag
where tag.Id = (
select max(tag2.Id)
from MyItem item2
join item2.Tags tag2
where item2.Id = item.Id
group by item2.Id
)
Related
Domain Model
I've got a canonical Domain of a Customer with many Orders, with each Order having many OrderItems:
Customer
public class Customer
{
public Customer()
{
Orders = new HashSet<Order>();
}
public virtual int Id {get;set;}
public virtual ICollection<Order> Orders {get;set;}
}
Order
public class Order
{
public Order()
{
Items = new HashSet<OrderItem>();
}
public virtual int Id {get;set;}
public virtual Customer Customer {get;set;}
}
OrderItems
public class OrderItem
{
public virtual int Id {get;set;}
public virtual Order Order {get;set;}
}
Problem
Whether mapped with FluentNHibernate or hbm files, I run two separate queries, that are identical in their Fetch() syntax, with the exception of one including the .First() extension method.
Returns expected results:
var customer = this.generator.Session.Query<Customer>()
.Where(c => c.CustomerID == id)
.FetchMany(c => c.Orders)
.ThenFetchMany(o => o.Items).ToList()[0];
Returns only a single item in each collection:
var customer = this.generator.Session.Query<Customer>()
.Where(c => c.CustomerID == id)
.FetchMany(c => c.Orders)
.ThenFetchMany(o => o.Items).First();
I think I understand what's going on here, which is that the .First() method is being applied to each of the preceding statements, rather than just to the initial .Where() clause. This seems incorrect behavior to me, given the fact that First() is returning a Customer.
Edit 2011-06-17
After further research and thinking, I believe that depending on my mapping, there are two outcomes to this Method Chain:
.Where(c => c.CustomerID == id)
.FetchMany(c => c.Orders)
.ThenFetchMany(o => o.Items);
NOTE: I don't think I can get subselect behavior, since I'm not using HQL.
When the mapping is fetch="join" I should get a cartesian product between the Customer, Order and OrderItem tables.
When the mapping is fetch="select" I should get a query for Customer, and then multiple queries each for Orders and OrderItems.
How this plays out with adding the First() method to the chain is where I lose track of what should be happening.
The SQL Query that get's issued is the traditional left-outer-join query, with select top (#p0) in front.
The First() method is translated into SQL (T-SQL at least) as SELECT TOP 1 .... Combined with your join fetching, this will return a single row, containing one customer, one order for that customer and one item for the order. You might consider this a bug in Linq2NHibernate, but as join fetching is rare (and I think you're actually hurting your performance pulling the same Customer and Order field values across the network as part of the row for each Item) I doubt the team will fix it.
What you want is a single Customer, then all Orders for that customer and all Items for all those Orders. That happens by letting NHibernate run SQL that will pull one full Customer record (which will be a row for each Order Line) and construct the Customer object graph. Turning the Enumerable into a List and then getting the first element works, but the following will be slightly faster:
var customer = this.generator.Session.Query<Customer>()
.Where(c => c.CustomerID == id)
.FetchMany(c => c.Orders)
.ThenFetchMany(o => o.Items)
.AsEnumerable().First();
the AsEnumerable() function forces evaluation of the IQueryable created by Query and modified with the other methods, spitting out an in-memory Enumerable, without slurping it into a concrete List (NHibernate can, if it wishes, simply pull enough info out of the DataReader to create one full top-level instance). Now, the First() method is no longer applied to the IQueryable to be translated to SQL, but it is instead applied to an in-memory Enumerable of the object graphs, which after NHibernate has done its thing, and given your Where clause, should be zero or one Customer record with a hydrated Orders collection.
Like I said, I think you're hurting yourself using join fetching. Each row contains the data for the Customer and the data for the Order, joined to each distinct Line. That is a LOT of redundant data, which I think will cost you more than even an N+1 query strategy.
The best way I can think of to handle this is one query per object to retrieve that object's children. It would look like this:
var session = this.generator.Session;
var customer = session.Query<Customer>()
.Where(c => c.CustomerID == id).First();
customer.Orders = session.Query<Order>().Where(o=>o.CustomerID = id).ToList();
foreach(var order in customer.Orders)
order.Items = session.Query<Item>().Where(i=>i.OrderID = order.OrderID).ToList();
This requires a query for each Order, plus two at the Customer level, and will return no duplicate data. This will perform far better than a single query returning a row containing every field of the Customer and Order along with each Item, and also better than sending a query per Item plus a query per Order plus a query for the Customer.
I'd like to update the answer with my found so that could help anybody else with the same problem.
Since you are querying the entity base on their ID, you can use .Single instead of .First or .AsEnumerable().First():
var customer = this.generator.Session.Query<Customer>()
.Where(c => c.CustomerID == id)
.FetchMany(c => c.Orders)
.ThenFetchMany(o => o.Items).Single();
This will generate a normal SQL query with where clause and without the TOP 1.
In other situation, if the result has more than one Customer, exception will be thrown so it won't help if you really need the first item of a series based on condition. You have to use 2 queries, one for the first Customer and let the lazy load do the second one.
Is there a way to achieve SQL like this with NHibernate ICriteria or QueryOver?
select *
from [BlogPost] b
inner join (select blogpost_id, count(*) matchCount
from [Tag]
where name in ('tag X', 'tag Y')
group by blogpost_id
) tagmatch
on tagmatch.blogpost_id = b.Id
order by tagmatch.matchCount desc
The aim is to rank blog posts by the number of matching tags so that a post with both tag X and tag Y comes above posts with just tag X.
I've got this so far:
DetachedCriteria
.For<Tag>("tag")
.Add(Restrictions.In(Projections.Property<Tag>(x => x.Name), tags.ToArray()))
.SetProjection(Projections.Group<Tag>(t => t.BlogPost))
.CreateCriteria("BlogPost")
.SetFetchMode("BlogPost", FetchMode.Eager)
.AddOrder(Order.Desc(Projections.RowCount()));
However, the resulting query doesn't join fetch BlogPost. Instead it returns just the ids, which leads to select n+1 when the BlogPosts are iterated.
public class BlogPost
{
...
ISet<Tag> Tags {get; set;}
}
public class Tag
{
BlogPost BlogPost { get; set; }
string Name { get; set; }
}
This looks like a similar issue.
Is this now possible with NHibernate 3?
If not, is there an alternative solution?
I can change schema & domain model if necessary. I don't want to use SQL or HQL if possible.
I know this question was put some time ago, but I want to do about the same thing, please take a look to my question here, and this guy here, maybe you can use the idea.
public class City
{
virtual public long Id { get; set; }
virtual public string Name { get; set; }
}
City table contains duplicated Names and I want to remove duplicates. I also want the results to be ordered by Id.
First I thought about the following query.
select distinct Name from City order by Id;
But this breaks with 'ORDER BY items must appear in the select list if SELECT DISTINCT is specified.' exception. After seeing http://weblogs.sqlteam.com/jeffs/archive/2007/12/13/select-distinct-order-by-error.aspx I think I should do:
select Name from City group by Name order by min(Id)
So my question is how can I do this query with QueryOver?
This is possible in ICriteria:
var list =
session.CreateCriteria<City>()
.SetProjection(Projections.Group("Name"))
.AddOrder(Order.Asc(Projections.Min("Id")))
.List<string>();
But this is not currently possible in QueryOver, because the .OrderBy(IProjection) overload is missing. Once the missing overload has been added it should look something like:
var list =
s.QueryOver<City>()
.Select(Projections.Group<City>(p => p.Name))
.OrderBy(Projections.Min<City>(c => c.Id)).Asc
.List<string>();
Note that the Projections overloads are there just now, so you can write the following (typesafe) query in ICriteria:
var list =
session.CreateCriteria<City>()
.SetProjection(Projections.Group<City>(c => c.Name))
.AddOrder(Order.Asc(Projections.Min<City>(c => c.Id)))
.List<string>();
So, what I've found is pretty simple...
var query = session.QueryOver<MyModel>()
// Conditions here
.OrderBy(m => m.GetThisDistinctField).Desc() // ...or Asc()...
.SelectList(m => m.SelectGroup(g => g.GetThisDistinctField));
var result = query.List<FieldDataType>().ToList();
return result;
To get an ordered query in queryover, start with a query that includes whatever criteria you need, but then add the SelectList/SelectGroup setup in order to get the distinct list. In other words, it's sort of like NHibernate can take a regular query, and then do special stuff to make it a select distinct query.
This is a solution I'm using in a current project I'm working on; I hope it helps someone else too.
Assuming the following:
public class Order
{
public virtual int OrderId {get;set}
public virtual ISet<Product> Products {get;set}
}
public class Product
{
public virtual int ProductId {get;set}
public virtual string ProductName {get;set}
}
How would you query using the criteria api so that only an order with a specific orderid is returned and its Product collection should also be filtered down to Products whose Name start with the lettter P?
Simplest approach is to use an alias:
var productIdToSelect = 9;
var crit = Session.CreateCriteria(typeof(Order));
crit.CreateAlias("Product", "prod");
crit.Add(Expression.Eq("prod.Id", productIdToSelect));
var result = crit.List<Order>();
I would go about this with a DetachedCriteria:
DetachedCriteria crit = DetachedCriteria.For<Order>();
crit.Add(Restrictions.Eq("OrderId",orderID);
crit.CreateCriteria("Products","products");
crit.Add(Restrictions.Like("products.ProductName","P%");
crit.List();
and then executing the criteria and getting the results.
I don't know the code you would have to write, but a point in the right direction:
http://www.nhforge.org/doc/nh/en/index.html#querycriteria-associations (14.4)
The key seems to be:
.SetResultTransformer(CriteriaUtil.AliasToEntityMap)
The documentation shows an example with cats and kittens.
Note that the kittens collections held by the Cat instances returned by the previous two queries are not pre-filtered by the criteria! If you wish to retrieve just the kittens that match the criteria, you must use SetResultTransformer(CriteriaUtil.AliasToEntityMap).
Set up a filter on the mapping of the collection.
<filter name="letterFilter" condition="ProductName like ':letterSupplied'"/>
Then before running the Order query enable the filter
session.EnableFilter("letterFilter").SetParameter("letterSupplied", "P%");
then run the query
Order ord = session.CreateCriteria<Order>().Add(Restrictions.IdEq(suppliedId)).UniqueResult<Order>();
Note that the single quotes in the filter definition may not be required and also i place the % symbol with the supplied parameter as i don't know how NH would react a filter like
<filter name="letterFilter" condition="ProductName like ':letterSupplied%'"/>
or
<filter name="letterFilter" condition="ProductName like :letterSupplied%"/>
Doman:
class Action
Products: IList of class ActionProducts:
Category: class Category
Products: IList of class Product
Now, I want this:
var products = from a in Session.Linq<Action>()
from ap in a.Products
from p in ap.Category.Products
where a.Name == name
select p;
And this Linq actually works but: 1. produces select for all tables instead of only Products 2. produces left outer joins, not inner 3. Distinct() on the query doesn't work (though ToList().Distinct() works).
Also can be done with SelectMany(a => a.Products).SelectMany(ap => ap.Category.Products) but it doesn't work at all with current NHibernate.Linq.
So I want to use ICriteria. But I can't see how do I return product, not action?
ICriteria criteria = Session.CreateCriteria(typeof(Action))
.Add(Expression.Eq("Name", name))
.CreateAlias("Products", "ap")
.CreateAlias("ap.Category.Products", "p")
.SomehowReturnMeOnly("p");
So how do I SomehowReturnMeOnly("p")? So that I can do
return criteria.List<Product>();
which will fail because ICriteria selects Actions, not Products?
I may consider HQL but I actually doesn't like string queries... Just for example, here's the HQL that works and produces exactly the SQL that I need:
IQuery query = Session.CreateQuery("select distinct p from Action a inner join a.Products as ap inner join ap.Category.Products as p");
return query.List<Product>();
Now, something similar can be done (keeping in mind that CreateAlias can only do 1 level) using
DetachedCriteria dq = DetachedCriteria.For<Action>()
.Add(Expression.Eq("Name", name))
.CreateAlias("Products", "ap")
.CreateAlias("ap.Category", "c")
.CreateAlias("c.Products", "p")
.SetProjection(Projections.Property("p.Id"));
ICriteria criteria = Session.CreateCriteria(typeof(Product))
.Add(Subqueries.PropertyIn("Id", dq));
return criteria.List<Product>();
This works and passes test, but produces "SELECT FROM products WHERE id in (subquery)" which may be even better (no DISTINCT required) but is not what I wanted to achieve. Seems like Criteria API is very, very restrictive. So we have:
HQL with string-query drawbacks
Criteria API with lots of restrictions and sometimes awful code to achieve simple results
NH-Linq which looks very promising but is incomplete now.
So I guess I'll stick with HQL until Linq is ready.
You need to use Projections, something like this:
ICriteria criteria = Session.CreateCriteria(typeof(Action))
.Add(Expression.Eq("Name", name))
.CreateAlias("Products", "ap")
.CreateAlias("ap.Category.Products", "p")
.SetProjection(Projections.Property("ap.Category.Products"))
.List<Product>();
Have a look at the nhibernate docs here for some examples.
Well, after thinking about Chris' answer... I tried this and it seemed to work:
ICriteria criteria = Session.CreateCriteria(typeof(Action))
.Add(Expression.Eq("Name", name))
.CreateAlias("Products", "ap")
.CreateAlias("ap.Category", "c")
.SetProjection(Projections.Distinct(Projections.Property("c.Products")));
Looks like NHibernate doesn't allow deep nesting of projection properties which is strange. And it doesn't work either, looking at generated SQL I see that it only selects
SELECT distinct c2_.Id as y0_ FROM ... Categories c2_ ...
i.e. it doesn't really fetch products, which makes my unit test to fail because returned list contains only nulls instead of Product instances.