[Geoprisma-dev] Attribute filtering

Tue Nov 2 10:06:19 EDT 2010

Hi Alexandre,

We don't need access control based on filters for this specific project, 
but it's still something that's on our wishlist. I see that as somewhat 
different from the attribute filtering widget, since this kind of access 
control would have to be implemented server-side (probably by putting 
some logic in the proxy) to offer any real security.

I started coding a first chunk of the widget, and will post to 
GeoExt-dev when I have something to show.

Thanks for your feedback,

Etienne

On 01/11/2010 8:12 AM, Alexandre Dube wrote:
> Etienne,
>
>   I totally agree that this could be a GeoExt.ux widget.  All the 
> features you mentioned about it have no direct dependence on 
> GeoPrisma.  A thread could be started on the GeoExt-DEV ML.
>
>   In you project, do you need any access control using filtering ?  
> For example, "restrict the data X to a given extent and a given 
> attribute having a specific value to the user Y".   If so, that could 
> be discussed here.
>
> Regards,
>
> Alexandre
>
>
> On 10-10-29 03:11 PM, Etienne Dube wrote:
>> 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
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Tel. :  514.313.5951 #1131
Courriel: etienne.dube at boreal-is.com