PG::GroupingError is raised when trying to group results

[cupofjoakim]

Just trying out lines from the readme when i came across this. Currently at 2.1.1 with this gem, running pg gem at 0.21.0 and postgres at 9.5. Am I doing something wrong?

results = PgSearch.multisearch(search_string).group(:searchable_type)
!! #<ActiveRecord::StatementInvalid: PG::GroupingError: ERROR:  column "pg_search_documents.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT  "pg_search_documents".* FROM "pg_search_documents" I...
                ^
: SELECT  "pg_search_documents".* FROM "pg_search_documents" INNER JOIN (SELECT "pg_search_documents"."id" AS pg_search_id, (ts_rank((to_tsvector('simple', coalesce("pg_search_documents"."content"::text, ''))), (to_tsquery('simple', ''' ' || 'anders' || ' ''')), 0)) AS rank FROM "pg_search_documents" WHERE (((to_tsvector('simple', coalesce("pg_search_documents"."content"::text, ''))) @@ (to_tsquery('simple', ''' ' || 'anders' || ' '''))))) AS pg_search_ce9b9dd18c5c0023f2116f ON "pg_search_documents"."id" = pg_search_ce9b9dd18c5c0023f2116f.pg_search_id GROUP BY "pg_search_documents"."searchable_type" ORDER BY pg_search_ce9b9dd18c5c0023f2116f.rank DESC, "pg_search_documents"."id" ASC LIMIT $1>

[coorasse]

Similar to #367