Support needed in st_as_sf() for '<dsn>|layername=<name>' output as obtained from qgis_process

[florisvdh]

If the output file and layer destination are prespecified in 'the QGIS way', i.e. like ogr:dbname=world.gpkg table=norway or ogr:dbname=world.gpkg table=norway (geom) these are recognized well by the qgis_process command. The latter adapts to the way these parameters are seen in the processing tool dialog (from which they can be copied as JSON and fed to qgis_process and to qgis_run_algorithm() in R).

In its output, qgis_process transforms this output specification into a <dsn>|layername=<name> syntaxis.

However when passing the same QGIS syntaxis either as a regular qgis_run_algorithm() argument [1], or as JSON (typically copied from the QGIS processing tool dialog) [2], st_as_sf() fails to read the <dsn>|layername=<name> syntaxis of the output element(s).

A reprex for [2] is given in #74 (comment).

A reprex for [1] is below:

library(qgisprocess)
#> Using 'qgis_process' at '/usr/bin/qgis_process'.
#> QGIS version: 3.22.3-Białowieża
#> Configuration loaded from '~/.cache/R-qgisprocess/cache-0.0.0.9000.rds'
#> Run `qgis_configure()` for details.
#> Using JSON for output serialization
library(sf)
#> Linking to GEOS 3.10.1, GDAL 3.4.0, PROJ 8.2.0; sf_use_s2() is TRUE

curl::curl_download(
  "https://github.com/qgis/QGIS/raw/master/resources/data/world_map.gpkg",
  "world_map.gpkg"
)

qgis_run_algorithm(
    "native:extractbyattribute",
    FIELD = "NAME",
    INPUT = "world_map.gpkg|layername=countries",
    OPERATOR = 0,
    VALUE = "Norway",
    OUTPUT = "ogr:dbname=world.gpkg table=norway",
    FAIL_OUTPUT = "ogr:dbname=world.gpkg table=except_norway"
  ) %>% 
  st_as_sf
#> Running /usr/bin/qgis_process --json run 'native:extractbyattribute' \
#>   '--INPUT=world_map.gpkg|layername=countries' '--FIELD=NAME' '--OPERATOR=0' \
#>   '--VALUE=Norway' '--OUTPUT=ogr:dbname=world.gpkg table=norway' \
#>   '--FAIL_OUTPUT=ogr:dbname=world.gpkg table=except_norway'
#> qt5ct: using qt5ct plugin
#> Error: Cannot open "world.gpkg|layername=except_norway"; The file doesn't seem to exist.

^{Created on 2022-01-31 by the reprex package (v2.0.1)}

Session info

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A suggested remedy will be added in PR #80.

Note: passing "world.gpkg|layername=except_norway" to qgis_process or (in R) qgis_run_algorithm() for the output is not a good option. It writes a GPKG named as world.gpkg|layername=except_norway.gpkg (which isn't a problem to st_as_sf(), then, but it's clearly not what we need).

[paleolimbot]

Since this is unique to qgis output as far as I know, we'll probably have to make a dedicated function for this in qgisprocess. I think strsplit(input, "|layername=", fixed = TRUE)[[1]] should do the trick?

[florisvdh]

we'll probably have to make a dedicated function for this in qgisprocess

Does the approach PR #80 already fix this for you, where it is done for st_as_sf()? :

https://github.com/paleolimbot/qgisprocess/blob/dca0591553c0829c435b0b56b293a5b8116a1e87/R/compat-sf.R#L27-L29

So, does the <dsn>|layername=<name> syntaxis also occur in other than vector outputs?

[paleolimbot]

Oh yeah! I thought I saw a test for that but didn't put together that you filed a separate issue.