Re: Sorting on a field that can have null values

[Theodan <da...@theodan.com>]

Chris Hostetter wrote:
> 
> 
> : If i rememebr correctly (you'll have to test this) sorting on a field
> : which doesn't exist for every doc does what you would want (docs with
> : values are listed before docs without)
> 
> : The actual behavior is different than described above. I modified
> : TestSort.java:
> 
> : The actual order of the results is: "ZJI". I believe this happens
> because
> : the field string cache 'order' array contains 0's for all the documents
> that
> : don't contain the field and thus sort first.
> 
> i guess wasn't precise enough in that old thread, what i ment was that not
> having a vlaue results in the docs sorting the same as if they had a value
> lower then the lowest existing value -- so they sort at the end of the
> list if you are doing a descending sort, and at the begining of the list
> if you do an ascending sort.  If you want to always have them come "last"
> regardless of order, there is a SortComparator for that purpose in Solr...
> 
> https://issues.apache.org/jira/browse/LUCENE-406
> http://svn.apache.org/viewvc/lucene/solr/trunk/src/java/org/apache/solr/search/MissingStringLastComparatorSource.java?view=log
> 
> 

But how can you use both the MissingStringLastComparatorSource and also your
own custom SortComparator (i.e. having a custom getComparable() method)?

I have tried the obvious, which was to make my custom SortComparator extend
MissingStringLastComparatorSource instead of SortComparator.  But then it
seems that my custom getComparable() method is ignored.  The sorting
framework doesn't seem to use the Comparables returned from my
getComparable() method to sort the results; instead, it seems to use the
ScoreDocComparator returned from the newComparator() method of
MissingStringLastComparatorSource.

FYI, my end goal is to be able to sort on a field called "AssetType".  Some
of the docs in the index may be missing this field (and I'd like those docs
to be sorted at the end of the results).  Furthermore, I need a custom
sorting order on the values in this "AssetType" field (first videos, then
articles, then images, etc.).

Here is my custom comparator, after changing it to extend
MissingStringLastComparatorSource (all that I changed was the "extends"
clause; the body remained the same):

======================================================================
private static class AssetTypeSortComparator extends
MissingStringLastComparatorSource /*SortComparator*/ {

	private static final Map ASSET_TYPE_ORDER_MAP = new HashMap();
	static {
		ASSET_TYPE_ORDER_MAP.put("Video", new Integer(0));
		ASSET_TYPE_ORDER_MAP.put("Article", new Integer(1));
		ASSET_TYPE_ORDER_MAP.put("Image", new Integer(2));
	}

	private static final Integer DEFAULT_ORDER = new Integer(3);

	protected Comparable getComparable(String termtext) {
		if (ASSET_TYPE_ORDER_MAP.containsKey(termtext)) {
			return (Integer)ASSET_TYPE_ORDER_MAP.get(termtext);
		}
		else {
			return DEFAULT_ORDER;
		}
	}

}
======================================================================

-Theo
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Re: Sorting on a field that can have null values

[Chris Hostetter <ho...@fucit.org>]

: But how can you use both the MissingStringLastComparatorSource and also your
: own custom SortComparator (i.e. having a custom getComparable() method)?
:
: I have tried the obvious, which was to make my custom SortComparator extend
: MissingStringLastComparatorSource instead of SortComparator.  But then it
: seems that my custom getComparable() method is ignored.  The sorting

well, yes that's true.  getComparable is an abstract method in
SortComparator so that subclasses of SortComparator can efinte how the
newComparator method will behave ... if you don't subcalss SortComparator
getComparable has no meaning.

(SortComparator is an instance of SortComparatorSource, but that doesn't
mean all other instances of SortComparatorSource -- like
MissingStringLastComparatorSource -- have any notion of APIs introduced in
SortComparator)

It didn't occur to me beofre that you were writing a custom SortComparator
... skimming the code for SortComparator and FieldCache briefly there
seems to be a limitation/feature (depending on how you look at it)
such that getComparable(null) is never called to decide what do do with
the null values -- it just assumes you want a null Comparable as well.

: FYI, my end goal is to be able to sort on a field called "AssetType".  Some
: of the docs in the index may be missing this field (and I'd like those docs
: to be sorted at the end of the results).  Furthermore, I need a custom
: sorting order on the values in this "AssetType" field (first videos, then
: articles, then images, etc.).

Frankly, My advice to you is to "encode" the AssetType field into some new
"SortableAssetType" field when you index your docs -- the code to do that
is not only easier to udnerstand then writiting a custom
SortComparator or SortComparatorSource but it's also going to be a lot
faster ... you do a little more work when indexing, and you make
searching/sortign a lot more efficient.


-Hoss


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