Add recursive helper methods for Group

Groups can be nested in arbitrary layers, so components listing groups and their subgroups need to be able to list all members recursively, or all sub-groups recursively.

PostgreSQL provides recursive CTEs.

added part::backend type::feature labels

unassigned @nik

changed the description

created merge request !852 (merged) to address this issue

mentioned in merge request !852 (merged)

 WITH recursive cte AS (
(
       SELECT "core_group_parent_groups"."to_group_id"
       FROM   "core_group_parent_groups"
       WHERE  "core_group_parent_groups"."from_group_id" = 16)
UNION ALL
          (
                     SELECT     "core_group_parent_groups"."to_group_id"
                     FROM       "core_group_parent_groups"
                     INNER JOIN "cte"
                     ON         "core_group_parent_groups"."from_group_id" = "cte"."to_group_id"))
SELECT          "core_group"."id",
                "core_group"."site_id",
                "core_group"."extended_data",
                "core_group"."school_term_id",
                "core_group"."name",
                "core_group"."short_name",
                "core_group"."group_type_id",
                "core_schoolterm"."id",
                "core_schoolterm"."site_id",
                "core_schoolterm"."extended_data",
                "core_schoolterm"."name",
                "core_schoolterm"."date_start",
                "core_schoolterm"."date_end"
FROM            "core_group"
INNER JOIN      "cte"
ON              "core_group"."id" = "cte"."to_group_id"
LEFT OUTER JOIN "core_schoolterm"
ON              (
                                "core_group"."school_term_id" = "core_schoolterm"."id")
WHERE           "core_group"."site_id" = 1
ORDER BY        "core_group"."short_name" ASC,
                "core_group"."name" ASC 

closed with merge request !852 (merged)

mentioned in commit b215f55d