问题
Dear NHibernate experts,
The following query gives me all my categories:
var result = Session.QueryOver(() => cat).List();
.. and by running this query, I get the ones selected (category_x_product table):
int productId = 11;
Category cat = null;
CategoryProduct cp = null;
var subQuery = QueryOver.Of(() => cp)
.Where(() => cp.ProductId == productId)
.Select(Projections.Distinct(Projections.Property(() => cp.CategoryId)));
result = Session.QueryOver(() => cat)
.WithSubquery
.WhereProperty(() => cat.Id).In(subQuery)
.List();
Any way to combine those two queries, so that I get all categories with a boolean value indicating which one was in fact "selected" in the CategoryProduct-query.
Map it to an entity like this, maybe?
CategorySelected
----------------
Category Category { get; set; }
bool IsSelected { get; set;
I've tried to find an answer to this, using QueryOver, but didnt succeed. Is this even possible in a "more or less" simple query? Any help is much appreciated. Thanks!
mikal
回答1:
One way to achieve that, is to create Conditional SELECT statement. In Case of SQL Server we would like to generate something like this
SELECT CASE CategoryId IN (.... subselect ) THEN 1 ELSE 0 END ...
But thanks to NHibernate, and abstract Querying API, we can create the query to be working in all supported DB dialects.
Let's try to create a draft of new solution. We will adjust the SubQuery
first
var subQuery = QueryOver.Of(() => cp)
.Select(Projections.Distinct(Projections.Property(() => cp.CategoryId)));
Now we will create the conditional statement
var isSelected = Projections.Conditional(
Subqueries.PropertyIn("Id", subQuery) // Category ID to be in the inner select
, Projections.Constant(1)
, Projections.Constant(0)
);
And we will inject that condition into the QueryOver
and use Transformers
to have correctly populated properties of the Category (including the virtual IsSelected
)
Category category = null
result = Session.QueryOver(() => cat)
// SELECT clause is now built
.SelectList(list => list
.Select(isSelected).WithAlias(() => category.IsSelected)
.Select(ca => ca.Id).WithAlias(() => category.Id)
... // all properites we would like to be populated
)
// Transform results into Category again
.TransformUsing(Transformers.AliasToBean<Category>())
.List<Category>();
And now, Our new IsSelected
property, which is not mapped, but only used for this SELECT (projections), is populated with the correct information.
NOTE: this approach is working, but the statements should be taken as a draft. Some adjustment could be needed in your case...
来源:https://stackoverflow.com/questions/18812892/nhibernate-queryover-retrieve-all-and-mark-the-ones-already-selected