[Qgis-developer] Memory data provider persistence

[Chris Crook]

Hi Even

Sorry .. lost wasn't quite correct.  The point is that my code for saving some users data and then reloading needs to take account of whether they have a field called fid or not if I want the list of fields unchanged after saving and reloading the data.

Either I have to record somewhere that the fid field is real (ie was in the original data set), and set the GML_EXPOSE_FID accordingly so that it appears in the reloaded data, or alternatively rename the users fields an a reversible way before I save the data so that none of the saved fields can be called fid (eg prefix all fields with "u_" for example), and set GML_EXPOSE_FID to NO.

At least that is my understanding of the way the driver works now?

Cheers
Chris 

-----Original Message-----
From: Even Rouault [mailto:even.rouault at mines-paris.org] 
Sent: Friday, 27 July 2012 7:59 a.m.
To: Chris Crook
Cc: qgis-developer at lists.osgeo.org; haubourg
Subject: Re: [Qgis-developer] Memory data provider persistence

Le jeudi 26 juillet 2012 21:41:57, Chris Crook a écrit :
> Hi Even
> 
> Thanks for the suggestion and code examples - very helpful. I haven't 
> followed up on this yet partly because this has prompted me to look at 
> other storage formats for the data (something I'd been putting off).
> 
> If I understand the GML driver documentation properly then I would 
> have to manage the possibility of the user having a field called FID, 
> as after I've saved the data, then reloaded with GMS_EXPOSE_FID set to 
> NO, that field will be lost?

If a user creates a 'fid' field, then that one will be used to set the fid attribute of the element. It will not be "lost" strictly speaking, but indeed if GML_EXPOSE_FID is set to NO, then it will not be visible.

> As you suggest, renaming the fields before saving and then after 
> reloading would fix handle this.

Not sure to understand you here : are you speaking about the mechanism for saving the old value of GML_EXPOSE_FID, setting GML_EXPOSE_FID=OLD, calling
OGROpen() and restoring the original value of GML_EXPOSE_FID ? If so, this will not change anything about the behaviour with a user field called 'fid'.

The intention here was not to interfere with other parts of QGIS that might manipulate GML files. For example, if a user opens a GML file through "Add Vector Layer", he might want to see the fid reported.

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