[Geoprisma-dev] Attribute filtering

[Etienne Dube]

Hi guys,

We have a new requirement in one of our projects, that involves 
attribute filtering on a vector layer. What's needed is a widget that 
allows the user to filter the features to be displayed by specifying an 
attribute, an operator and a comparison value. One or more of these 
criteria could be combined using logical operators (AND/OR) to create 
more complex queries; eventually multiple filtering expressions could be 
combined by forming a tree of conditions, similar to what the OGC Filter 
Encoding standard describes. Such a feature has already been discussed 
previously (see ticket http://trac.osgeo.org/geoprisma/ticket/38).

I spent some time defining the problem and exploring what tools could be 
used to solve it. A few thoughts:

- We could offer two different modes for this widget:

     1. A preconfigured mode, in which the choices of attributes (and 
possibly operators) to filter on, along with the logical operators used 
to combine these criteria, would be defined in the widget's 
configuration. Different preconfigured query templates could be defined 
in this way, for every layer the widget applies to. The user would only 
have to fill in the values to filter on.

     2. An "expression builder" mode. In this case, the choice of 
attributes, operators and any logical combination (AND/OR) or criteria 
would be left to the user, and a GUI panel would be provided to build 
these expressions. This mode mostly targets GIS specialists and other 
power users who have an understanding of boolean logic, in order to 
fulfill various free-form querying needs.

- Initially we could provide support for vector layers only. OpenLayers 
already provides the OpenLayers.Filter classes to build the filtering 
expressions, and is able to do filtering on the client side. However, 
since it's not a particularly efficient way of doing things, a way to 
pass the filter expression to the service would be desirable. As Yves 
mentioned in ticket #38, FeatureServer offers basic attribute filters as 
criteria passed in key/value pairs in a GET request, but this support is 
quite limited: it seems that only 1 criteria per attribute is accepted, 
there's no way to combine multiple criteria using AND/OR operators (AND 
is the default when specifying a filter for more than 1 attribute) and 
only the PostGIS data source is supported for attribute filters. Short 
of extending FS and its protocol, a solution could be to add a WFS proxy 
and pass a Filter Encoding expression in the request (as you may know, 
OL already has the logic to generate an ogc:Filter element from 
OpenLayers.Filter objects).

- Support for filtering WMS layers (issued from vector sources) would be 
a good idea too. I believe it's possible by passing an ogc:Filter as 
part of a SLD in the GetMap request.

- OpenGeo developed a QueryPanel widget (it's in their GXP repository) 
that offers similar functionality; see 
http://workshops.opengeo.org/geoext/more_ogc/app.html. I've not been 
able to make it work yet, and from what I see it can only apply filters 
on WFS and WMS layers. Nevertheless, this could be a good starting point.

- For the moment we can settle with text boxes for entering filter 
values, but it would be interesting to offer other control types (date 
picker, selection list). Another cool feature to have would be the 
ability to populate such selection lists with domain values (e.g. coming 
from a catalog table in the DB). However I'm not aware if any feature 
service (be it FeatureServer, WFS or MapFish Server) can do that kind of 
stuff, and I think it wouldn't be extremely elegant to have to establish 
an off-the-band connection to the data source for retrieving those 
domain value lists.

I think that the widget could be developed as a GeoExt ux first, since 
it's usefulness goes beyond the scope of GeoPrisma. I'm writing here to 
have some initial feedback, and I'm open to continue the discussion on 
the GeoExt dev mailing list if you feel it would be more relevant there.

Ideas, suggestions and corrections are welcome.

Etienne

-- 
Etienne Dubé
Consultant / Développeur

Boréal Informations Stratégiques
101, Du Moulin, bureau 202-A
Magog (Québec)
J1X 4A1

Tel. :  514.313.5951 #1131
Courriel: etienne.dube at boreal-is.com