Collection cache_key raises PG::GroupingError with select in query

[plguo]

ActiveRecord::Relation#cache_key raises PG::GroupingError in PostgreSQL when there is select in the query. I tested the same query in sqlite, and it didn't raise any error.

irb(main):001:0> Post.all.cache_key
   (14.5ms)  SELECT COUNT(*) AS size, MAX("posts"."updated_at") AS timestamp FROM "posts"
=> "posts/query-00644b6a00f2ed4b925407d06501c8fb-5-20160112214758318542"

irb(main):002:0> Post.all.select(:title).cache_key
   (1.7ms)  SELECT "posts"."title", COUNT(*) AS size, MAX("posts"."updated_at") AS timestamp FROM "posts"
ActiveRecord::StatementInvalid: PG::GroupingError: ERROR:  column "posts.title" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "posts"."title", COUNT(*) AS size, MAX("posts"."updat...
               ^
: SELECT "posts"."title", COUNT(*) AS size, MAX("posts"."updated_at") AS timestamp FROM "posts"

The problem seems to be fixed by adding unscope(:select)

irb(main):003:0> Post.all.select(:title).unscope(:select).cache_key
   (2.1ms)  SELECT COUNT(*) AS size, MAX("posts"."updated_at") AS timestamp FROM "posts"
=> "posts/query-00644b6a00f2ed4b925407d06501c8fb-5-20160112214758318542"