[Qgis-developer] Field Calculator "permanence"

[Matthias Kuhn]

On 07.11.2014 17:25, 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 ;-)

we could still add materialized virtual fields :-)

>
> 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.
... or use layer -> save as before sharing it. That will make a
permanent copy of the values at the time of saving.

Matthias

>
> 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:    +27-86-6115323     www.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:    +27-86-6115323     www.geograph.co.za
>> ===========================================
>>
>>
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>
>
>
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