Ayende @ Rahien

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NHibernate Filters

One of the more interesting ability of NHibernate is to selectively filter records based on some global filters. This allow us to very easily create global where clauses that we can flip on and off at the touch of a switch.

Let us take a look at see what I mean.

We define the filter effectiveDate:

<filter-def name="effectiveDate">
	<filter-param name="asOfDate" type="System.DateTime"/>
</filter-def>

A filter definition is most commonly just a set of parameters that we can define, which will later be applied to in the appropriate places. An example of an appropriate place would be Post.PostedAt, we don’t want to show any post that was posted at a later time than the effective date. We can define this decision in the mapping, like this:

<class name="Post"
		   table="Posts">
		<id name="Id">
			<generator class="identity"/>
		</id>
		
		<property name="Title"/>
		<property name="Text"/>
		<property name="PostedAt"/>
		
		
		<filter name="effectiveDate"
						condition=":asOfDate >= PostedAt"/>
	</class>

And now we can start play:

s.CreateCriteria<Post>()
	.SetMaxResults(5)
	.List();

s.EnableFilter("effectiveDate")
	.SetParameter("asOfDate", DateTime.Now);

s.CreateCriteria<Post>()
	.SetMaxResults(5)
	.List();

Who do you think this will generate?

Well, the first query is pretty easy to understand:

image

But the second one is much more interesting:

image

We have selectively applied the filter so only posted posted after the 16th can be seen.

This is a very powerful capability to have, since we can use this globally, to define additional condition. For that matter, we can apply it in multiple places, so comments would also be so limited, etc.

For that matter, we can also put filters on associations as well:

<set name="Comments"
	 table="Comments">
	<key column="PostId"/>
	<one-to-many class="Comment"/>
	<filter name="effectiveDate"
					condition=":asOfDate >= PostedAt"/>
</set>

And trying to access the Comments collection on a Post would generate the following SQL when the filter is active:

image

Nice, isn’t it?

Comments

Frank Quednau
05/04/2009 08:26 AM by
Frank Quednau

Hi Ayende,

would this be the right feature to leverage when implementing data restrictions based on user privileges?

Erik van Brakel
05/04/2009 12:05 PM by
Erik van Brakel

Wow, this is one feature I didn't know of. Thanks!

Ayende Rahien
05/04/2009 12:31 PM by
Ayende Rahien

Frank,

Depending on what you are doing.

I would tend to use Rhino Security for such tasks

Dmitry
05/04/2009 02:31 PM by
Dmitry

Filters are certainly very useful especially when dealing with records that expire or can have a temporary invalid/restricted state.

What I really don't like is that Session.Get and Session.Load commands ignore filters. Session.Load is not much of a problem because it is only being used when it is known that the record exists. But I always end up reimplementing the Get() method in the repository using a query to make sure developers do not accidentally load an invalid record and probably ending up losing some optimization benefits.

zoidbeck
05/11/2009 08:53 AM by
zoidbeck

Hi Ayende,

is it possible to use this filter with properties residing deeper in the objectgraph, for example on a
<many association within the filtered bag?

something like this, where Edits would be a mapped <many-to-one holding the timestamp:

<filter= Edits.EditTS"/>

zoidbeck
05/11/2009 08:59 AM by
zoidbeck

Sorry, that did not work. should have masked the xml.

Here is the example in pure text:

class blog

has a list of comments

class comment

has a list of edits

class edit

has a edit-timestamp property

filter:

name="effectiveDate"

condition=":asOfDate >= Edits.EditTS"

would this be possible using filters?

Comments have been closed on this topic.