For the most part, working with QGIS layers has been a pleasure, but I've run into some difficulties recently when trying to create a new vector layer in the SQLite database I've been using.
I've created a new database with the
QgsVectorFileWriter like this:
writer = QgsVectorFileWriter( "/path/to/db/testdb.sqlite", "utf-8", myFields, QgsWkbTypes.PointZ, QgsCoordinateReferenceSystem( 4326, QgsCoordinateReferenceSystem.EpsgCrsId), "SQLite", ["SPATIALITE=YES"])
And noticed that QGIS creates a default new table called "testdb" (named after the database) that works fine to load and save as to a layer.
I want to create a new layer with a new table now in the same database, so I went over to the docs at https://qgis.org/api/classQgsVectorFileWriter.html.
I saw there was an
ActionOnExistingFile parameter in the constructor to create a new layer & table, only to be disappointed when I noticed this parameter had been mysteriously dropped in the python implementation of the
QgsVectorFileWriter, according to the PyQGIS docs.
Feeling hopeful, I threw in an
action=QgsVectorFileWriter.CreateOrOverwriteLayer keyword, and even tried typing out all 11 arguments to avoid the keyword argument, but no luck.
# Doesn't work writer = QgsVectorFileWriter(... action=QgsVectorFileWriter.CreateOrOverwriteLayer)
I did see that the python
writeAsVectorFormat method had an option to specify the new table action, so I went ahead and wrote up something like this, using the
options = QgsVectorFileWriter.SaveVectorOptions() options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer options.layerName = "testlayer" writer = QgsVectorFileWriter.writeAsVectorFormat( layer, "/path/to/db/testdb.sqlite", options)
But... The layer argument assumes I already have a valid layer from a valid data source. I need to create the source first before I can create the layer though, right?
Using a memory layer...?
If one can't create the data source first, I thought maybe loading a layer with a temporary "memory" provider and then writing it to the database would be an option. I'm not sure how I would specify the other parameters that I need for the vector layer (such as the fields and the CRS) using this:
layer = QgsVectorLayer("PointZ", "testlayer", "memory")
and then saving it to the database. I'm not even sure whether this method would work, but it seems a little hacky to me in any case. But I could be wrong...