问题
I am currently dynamically constructing queries like so:
QueryOver<Base, Base> q = QueryOver.Of<Base>();
if (foo != null) q = q.Where(b => b.Foo == foo);
// ...
Now there are multiple mapped subclasses of Base
(e. g. Derived
) which I want to filter on, basically something like:
if (bar) q = q.Where(b => b is Derived); // does not work
or:
if (bar) q = q.Where(b => b.DiscriminatorColumn == 'derived'); // dito
How do I best achieve that, preferably - but not neccessarily - in a type-safe way? Can this be done using LINQ, too?
回答1:
This is not intuitive, but the following should work fine (QueryOver):
if (bar) q = q.Where(b => b.GetType() == typeof(Derived));
I'm not sure about a way to do this in LINQ-to-NH.
回答2:
The general QueryOver
, asking for a subtype
, would look like this:
Base alias = null;
var query = session.QueryOver<Base>(() => alias);
// this statement would be converted to check of the discriminator
query.Where(o => o is Derived);
var list = query.List<Derived>();
But this would result in a statement, expecting that discirminator is "MyNamespace.Derived". If this si not the case, we can use this approach:
Base alias = null;
var query = session.QueryOver<Base>(() => alias);
// here we can compare whatever value, we've used as discriminator
query.Where(Restrictions.Eq("alias.class", "Derived"));
var list = query.List<Derived>();
Here we are using the feature of NHibernate: ".class" which does return the value of discriminator
More details could be found here:
- 17.1.4.1. Alias and property references
来源:https://stackoverflow.com/questions/21677196/how-can-queryover-be-used-to-filter-for-a-specific-class