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Using QGIS Python API, I want to avoid updating geometry with an UPDATE sql command.

Why? I'm using a QSqlRelationalTableModel (see model-view programming if curious) and there are built-in editing strategy I want to benefit.

My issue is that for geometry, I should update the model content with a Spatialite geometry blob (specs).

I didn't found any blob functions in Spatialite for geometry, so I could do function_transform_geometry_to_spatialite_blob(MakePoint(0, 0, 4326)) and it returns a blob with geometry embedded as a text I can use to update my Geometry field within the model. Are there any I missed?

Maybe, it's not the right approach. You may have another suggestions/opinions.

PS: I know about http://blog.vitu.ch/10112013-1201/qgis-relations and I've seen this discussion but it does not seem as flexible as QSqlRelationalTableModel combined with QDataWidgetMapper (but I could be wrong)

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    You could make calls to the underlying spatialite functions (in C++, rather than SQL). I'd caution that if you do that, you are working around QGIS which may cause other issues later. Perhaps making this less abstract would help - show a small, executable example and say what you want to do instead. – BradHards Nov 21 '15 at 0:31
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It was obvious and I didn't read well the RTFM. See this discussion on the Spatialite mailing list.

The answer is:

SELECT ST_GeomFromText('POINT(1 2)', 4326);

or

SELECT MakePoint(1, 2, 4326);

To make result as a text, do:

SELECT hex(MakePoint(1, 2, 4326));

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