01_desired.sql

WITH RECURSIVE search_tree(id, path) AS (
  SELECT id, ARRAY[id]
  FROM services
  WHERE id = 42
UNION ALL
  SELECT services.id, path || services.id
  FROM search_tree
  JOIN services ON services.service_id = search_tree.id
  WHERE NOT services.id = ANY(path)
)
SELECT id FROM search_tree;

02_ruby.rb

puts \
  Arel::SelectManager.new.with(:recursive, # TODO ADD: (id, path)
    Arel::Nodes::As.new(search_tree,
      Arel::SelectManager.new(arel_table)
        .project(arel_table[primary_key], Arel::Nodes::NamedFunction.new("ARRAY", [arel_table[primary_key]])) # TODO change: ARRAY[id]
        .where(base_case)
      .union(Arel::SelectManager.new(arel_table)
        .project(arel_table[primary_key], Arel::Nodes::InfixOperation.new("||", search_tree[:path], arel_table[primary_key]))
        .join(search_tree).on(arel_table[ancestry_parent_column].eq(search_tree[primary_key]))
        .where(arel_table[primary_key].eq(Arel::Nodes::NamedFunction.new("ANY", [search_tree[:path]])).not)
      )
    )
  ).from(search_tree).project(search_tree[primary_key]).to_sql

03_output.sql

WITH RECURSIVE "search_tree" AS (
  SELECT "services"."id", ARRAY("services"."id")
  FROM "services"
  WHERE "services"."id" = 42
UNION
  SELECT "services"."id", "search_tree"."path" || "services"."id"
  FROM "services"
  INNER JOIN "search_tree" ON "services"."service_id" = "search_tree"."id"
  WHERE NOT ("services"."id" = ANY("search_tree"."path"))
)
SELECT "search_tree"."id" FROM "search_tree"