[QGIS-Developer] Editing buffer and commitChanges order

[Matthias Kuhn]

Hi Alessandro,

This problem also applies to editing multiple layers in parallel
(especially with foreign keys), that was one of the drivers to introduce
transaction editing.

For individual layers, I think we can keep track of all the changes in
the local edit buffer. For issue 16935, I think the order wouldn't
actually change much, it would still fail, just in a different order?
One thing there is, the commands could be sent in a single transaction
(which doesn't necessarily need to be managed by QgsTransaction).

On the other hand, I think splitFeature should be using
QgsVectorLayer.createFeature() which should take care of unique
constraints detected on the provider. If I'm not mistaken, this works
fine on postgres, possibly there's an issue with unique constraint
detection in sqlite based providers?

Cheers

Matthias


On 11/20/2017 01:02 PM, Alessandro Pasotti wrote:
> Hi,
>
> I'm facing a bug [1] that led me to a potential issue in the editing
> buffer implementation: when commitChanges is called, different
> operations (change geometry, change attrs, add features, delete
> features ...) are grouped and executed in a fixed order.
>
> So, if I wanted for example insert a feature and (after the insert has
> been successful)
> change the geometry of another feature I cannot do that because all
> geometry editing operations are grouped and executed before all the
> insert feature operations are executed [2]
>
> This issue can lead to data loss in several scenarios, one is the bug
> I'm working on here but there are more: for example when you want to
> merge features you cannot delete features after you have successfully
> created the merged one, just because all delete operations are
> performed before the insert ones.
>
> I guess that by grouping related commands together we probably wanted
> to make it easier to optimize the operations in the back-end.
>
> Wrapping the commitChanges into transactions would help with DB-like
> backends that support it (I think that the implementation is not yet
> completed), but I'm still convinced that the order of edit commands
> within an editing buffer should be respected (that does not mean that
> certain commands cannot be grouped if the programmer choose to do so).
>
> I'd like to hear your opinion about the possible solutions to this
> problem.
>
>
> [1] https://issues.qgis.org/issues/16935
> [2]
> https://github.com/qgis/QGIS/blob/master/src/core/qgsvectorlayereditbuffer.cpp#L300
>
>
> -- 
> Alessandro Pasotti
> w3:   www.itopen.it <http://www.itopen.it>
>
>
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