[Qgis-developer] Field Calculator "permanence"

[Andreas Neumann]

Hi Zoltan,

If you remove and re-add the shapefile you will of course loose the 
virtual fields, as they are stored with the project and not with the data.

However, if you save a layer definition file along with the shapefile 
and re-add that, they are preserved. So you can just store the layer 
definition file along with the shape and you are fine to share the data 
with your colleagues, preserving virtual fields ;-)

Andreas

On 07.11.2014 17:32, Zoltan Szecsei wrote:
> 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
>>> ===========================================
>>>
>>>
>>> _______________________________________________
>>> Qgis-developer mailing list
>>> Qgis-developer at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/qgis-developer
>>
>>
>>
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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:    +27-86-6115323www.geograph.co.za
> ===========================================
>
>
> _______________________________________________
> Qgis-developer mailing list
> Qgis-developer at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/qgis-developer

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