Strange difference between .size and .count when grouping SQL query - ruby-on-rails

Question

I have scope in Usage model:
# == Schema Information
# id :uuid
# service :string
# organization_id :uuid
# document_id :uuid
# created_at :datetime
class Usage < ApplicationRecord
belongs_to :document
scope :grouped, -> {
joins(document: :user)
.select("COUNT(usages.document_id), usages.service, usages.document_id, documents.subject, MAX(usages.created_at)")
.group('service', 'documents.subject', 'document_id', "CONCAT(users.first_name, ' ', users.last_name)", 'usages.created_at::date')
.order("MAX(usages.created_at) DESC")
}
...
When in the rails console I do result = Usage.all.grouped, result:
Usage Load (2.5ms) SELECT COUNT(usages.document_id), usages.service, usages.document_id, documents.subject, MAX(usages.created_at) FROM "usages" INNER JOIN "documents" ON "documents"."id" = "usages"."document_id" INNER JOIN "users" ON "users"."id" = "documents"."user_id" GROUP BY "usages"."service", documents.subject, "usages"."document_id", CONCAT(users.first_name, ' ', users.last_name), usages.created_at::date ORDER BY MAX(usages.created_at) DESC LIMIT $1 [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Usage id: nil, service: "document", document_id: "f3455d09-0681-46d0-bf2d-54bb3286089b">, #<Usage id: nil, service: "document", document_id: "db3303de-f2ac-4611-8603-da70847e7eeb">, #<Usage id: nil, service: "document", document_id: "642b9fa8-e72e-4bb8-9551-d30f92bf7e67">]>
Then, to expand this query into a hash with the parameters I need, I call the .size method on this scope.
result.size
(1.7ms) SELECT COUNT(*) AS count_all, "usages"."service" AS usages_service, documents.subject AS documents_subject, "usages"."document_id" AS usages_document_id, CONCAT(users.first_name, ' ', users.last_name) AS concat_users_first_name_users_last_name, usages.created_at::date AS usages_created_at_date FROM "usages" INNER JOIN "documents" ON "documents"."id" = "usages"."document_id" INNER JOIN "users" ON "users"."id" = "documents"."user_id" GROUP BY "usages"."service", documents.subject, "usages"."document_id", CONCAT(users.first_name, ' ', users.last_name), usages.created_at::date ORDER BY MAX(usages.created_at) DESC
=> {
["document", "Alena Cartwright II welcome a Gorgon", "f3455d09-0681-46d0-bf2d-54bb3286089b", "Test Test", Tue, 09 Aug 2022]=>1,
["document", "Thanh Harber Sr. view a Ankylosaurus", "db3303de-f2ac-4611-8603-da70847e7eeb", "Test2 Test", Mon, 08 Aug 2022]=>1,
["document", "Rolando Langosh DO bust a Clay Golem", "642b9fa8-e72e-4bb8-9551-d30f92bf7e67", "Test3 Test", Sun, 07 Aug 2022]=>1
}
As we can see, when the .size method is called, another SELECT query comes in.
According to the documentation - .size returns the size of the collection by executing a SELECT COUNT(*) query if the collection hasn't been loaded, and calling collection.length if it has.
But at the same time, I get an error if I try to call the .count method on this scope.
result.count
(1.4ms) SELECT COUNT(COUNT(usages.document_id), usages.service, usages.document_id, documents.subject, MAX(usages.created_at)) AS count_count_usages_document_id_usages_service, "usages"."service" AS usages_service, documents.subject AS documents_subject, "usages"."document_id" AS usages_document_id, CONCAT(users.first_name, ' ', users.last_name) AS concat_users_first_name_users_last_name, usages.created_at::date AS usages_created_at_date FROM "usages" INNER JOIN "documents" ON "documents"."id" = "usages"."document_id" INNER JOIN "users" ON "users"."id" = "documents"."user_id" GROUP BY "usages"."service", documents.subject, "usages"."document_id", CONCAT(users.first_name, ' ', users.last_name), usages.created_at::date ORDER BY MAX(usages.created_at) DESC
Traceback (most recent call last):
1: from (irb):5
ActiveRecord::StatementInvalid (PG::UndefinedFunction: ERROR: function count(bigint, character varying, uuid, character varying, timestamp without time zone) does not exist)
LINE 1: SELECT COUNT(COUNT(usages.document_id), usages...
What is the difference between using .size and .count in this example?
Why does the .count method wrap the existing SQL COUNT method with another COUNT?

Answer 1

From the docs;
If count is used with Relation#select, it will count the selected
columns:
# Person.select(:age).count
SELECT COUNT("people"."age") FROM "people"
So, if your query happens to have a SELECT, then it'll wrap whatever it contains within COUNT (see select_values)
By definition when using size it'll default to count(:all) unless the object is loaded. So you anyway end up using count.