Article.joins(:themes => [:users]).where(["articles.user_id != ?",current_user.id]).order("Random()").limit(15).uniq

并给我错误

PG::Error: ERROR:  for SELECT DISTINCT,ORDER BY expressions must appear in select list
LINE 1: ...s"."user_id" WHERE (articles.user_id != 1) ORDER BY Random() L...

当我更新原来的查询

Article.joins(:themes => [:users]).where(["articles.user_id != ?",current_user.id]).order("Random()").limit(15)#.uniq

所以错误消失了…在MysqL .uniq中,在Postgresql中没有.存在任何替代品？