[sensorthings] Support feature expansion (Request in QGIS)

[qgis-bot]

Request for documentation

From pull request qgis/QGIS#56960
Author: @nyalldawson
QGIS version: 3.38

[sensorthings] Support feature expansion

PR Description:

This change allows SensorThings entities to be expanded to contain their related child feature attributes, exposing the relational SensorThings model as a traditional "flat" GIS-friendly table structure.

Eg when selecting Location entities, you can now opt to expand to "Things > Datastreams > Observations". This would result in multiple "stacked" point location features, one corresponding to each observation, with the attributes for each point feature containing the location, thing, datastream and observation attributes.

Best used combined with some extent, feature limit, or custom filter option, as this can otherwise result in very heavy
requests to the backend service! There's also an option to limit then number of child features returned when expanding, and we default to a very conservative amount here in order to reduce load on services. (I.e. users need to explicitly "opt in" to fetch large amounts of features)

Here's an example of a feature returned when the expansion option out to "Location > Thing > Datastream" is used. Note how the "Thing" and "Thing_Datastream" attributes are appended on to the base "Location" attributes:

Fixes qgis/QGIS#56805

Commits tagged with [need-docs] or [FEATURE]