This blog shows us how we can apply a filter onto the class definition:-
<class name="Domain.Model.BlogPost, Domain.Model" table="Posts" where="(IsLive=1)"> ... </class>However be warned this will always return blog posts that are live. Sometimes you may want to get blog posts that are not live. If you use a filter this way then you will never be able to return isLive=0 or ALL blogs regardless of the flag.
Fortunately NHibernate allows us to switch filters on/off at a flick of a switch, add this to your mappings file:-
<filter-def name="BlogFlag"> <filter-param name=":isLive" type="System.Int"/> </filter-def> <class name="Domain.Model.BlogPost, Domain.Model" table="Posts"> <filter name="BlogFlag" condition="(isLive=:flag)"/> ... </class>Using the flag could not be simpler, just set the flag before you query:-
//return live blogs session.EnableFilter("BlogFlag").SetParameter("isLive", 1); session.QueryOver<MyEntity>(); //return blogs that are not live session.EnableFilter("BlogFlag").SetParameter("isLive", 0); session.QueryOver<MyEntity>(); //return all blogs session.QueryOver&t;MyEntity>();Now one question you are asking is why would I use filters? Why wouldn't I just use
//Use a where clause! Isn't this more sensible? session.QueryOver<MyEntity>() .Where(w => w.IsLive == isLive);However one of the unknown undocumented features of filters is that the database column IsLive, DOES not need to mapped to a fully mapped property.
The second reason is that if I ALWAYS want just live Blog posts therefore adding where="(IsLive=1)"> to the class definition makes sense as the developer will does not need to remember to add the where clause for EVERY query.
In the next blog post I will show you how to use filters on collections.