[Qgis-developer] Field Calculator "permanence"

[Zoltan Szecsei]

Hi Andreas,
I'll fiddle with the Postgis idea - thanks.
The orange part below:
I closed the shapefile, deleted from legend and re-opened it - but could 
not see my virtual field, nor the expression I used to create it.
Have I missed a trick somewhere?

Regards,
Z

On 2014/11/07 18:25, Andreas Neumann wrote:
> Hi,
>
> What you probably ask for are database triggers combined with storage. 
> This is not what virtual fields are for. If they were stored - they 
> wouldn't be called virtual ;-)
>
> You can do that f.e. with Postgis. With shapefiles not. However, if 
> you share the project with the other PC, the virtual fields work fine.
>
> Andreas
>
> On 07.11.2014 17:21, Zoltan Szecsei wrote:
>> Hi Matthias,
>> Thanks for the quick answer.
>> The virtual field acts just like I want it to, except that it does 
>> not permanently store the column in the attribute table.
>> I tried getting clever by then adding a second 'Field Calculation' to 
>> update the existing 'Sheet' field from the virtual field, but that 
>> won't do it either.
>>
>> So, for me there are two shortcomings:
>>
>>  1. To be able to store and recall the 'Expression' and not lose it
>>     after closing the Field Calculator
>>  2. To be able to add the virtual fields that you create, into the
>>     attribute table.
>>  3. (And then of course to re-open the Shape file on another PC and
>>     have these Field Calculations still functional)
>>
>> Any further possibilities I can try?
>>
>> Regards & thanks again,
>> Zoltan
>>
>>
>> On 2014/11/07 18:01, Matthias Kuhn wrote:
>>> Hi Zoltan,
>>>
>>> Since QGIS 2.6 this possibility exists. It is called "Virtual Field".
>>> You must not create the attribute when creating the shapefile. Just open
>>> the field calculator, create a new field in there and check the "virtual
>>> field" checkbox.
>>>
>>> Regards,
>>> Matthias
>>>
>>> On 07.11.2014 16:54, Zoltan Szecsei wrote:
>>>> Hi,
>>>> I'm using QGIS 2.6.0 on Win 7 x64 - installed via OSGEO4w
>>>>
>>>> Is there a way to store your field calculator expression to always
>>>> process and update that specific field, should geometry be edited or
>>>> created for that Layer?
>>>>
>>>> Let's say I want to create a blank POINT shape file, with a text
>>>> attribute called "Sheet".
>>>> Whenever a POINT is created or moved, I want this field to be
>>>> dynamically updated with:
>>>>
>>>>       tostring(abs(toint($y)))+tostring(abs(toint($x)))
>>>>
>>>> For testing thus far have:
>>>>
>>>>    * created 2 points
>>>>    * Opened the field calculator, clicked 'Update Existing Field' and
>>>>      put in the expression.
>>>>    * The two existing points now have the correct TEXT values in the
>>>>      Sheet field
>>>>    * I close the Field calculator
>>>>    * Create a new point in my layer
>>>>    * open the attribute table to find that the 'Sheet' field for this
>>>>      new point is blank
>>>>    * I open the Field Calculator and I cannot even find a way to call
>>>>      back the previous expression I used.
>>>>
>>>>
>>>> Am I using the correct QGIS functionality to achieve what I need to,
>>>> and if so, where am I going wrong?
>>>>
>>>> Thanks & regards,
>>>> Zoltan
>>>>
>>>>
>>>> -- 
>>>>
>>>> ===========================================
>>>> Zoltan Szecsei PrGISc [PGP0031]
>>>> Geograph (Pty) Ltd.
>>>> GIS and Photogrammetric Services
>>>>
>>>> P.O. Box 7, Muizenberg 7950, South Africa.
>>>>
>>>> Mobile: +27-83-6004028
>>>> Fax:    +27-86-6115323www.geograph.co.za
>>>> ===========================================
>>>>
>>>>
>>>> _______________________________________________
>>>> Qgis-developer mailing list
>>>> Qgis-developer at lists.osgeo.org
>>>> http://lists.osgeo.org/mailman/listinfo/qgis-developer
>>
>>
>> -- 
>>
>> ===========================================
>> Zoltan Szecsei PrGISc [PGP0031]
>> Geograph (Pty) Ltd.
>> GIS and Photogrammetric Services
>>
>> P.O. Box 7, Muizenberg 7950, South Africa.
>>
>> Mobile: +27-83-6004028
>> Fax:    +27-86-6115323www.geograph.co.za
>> ===========================================
>>
>>
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>
>
>
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-- 

===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
GIS and Photogrammetric Services

P.O. Box 7, Muizenberg 7950, South Africa.

Mobile: +27-83-6004028
Fax:    +27-86-6115323     www.geograph.co.za
===========================================

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