CHECK constraints from the database can specify accepted lists of values or numeric ranges. They can be represented in QGIS as value map or range widgets, respectively.
I don't have any immediate plans for adding support for this to the plugin. But if you have something in mind, feel free to make a pull request. I will most likely add it in that case.
CHECK constraints from the database can specify accepted lists of values or numeric ranges. They can be represented in QGIS as value map or range widgets, respectively.