Distinct select bug when sorting on joined record

[calebfaruki]

irb(main):022:0> Requirement.filter(params['where']).sort(customer: {name: {asc: :nulls_last}}).distinct_on(:id)
Traceback (most recent call last):
ActiveRecord::StatementInvalid (PG::InvalidColumnReference: ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions)
LINE 1: SELECT DISTINCT ON ( "requirements"."id" ) "requirements".* ...

[malomalo]

So PostgresQL states:

DISTINCT ON ( expression [, ...] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. [...] Note that the “first row” of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first.

so.... looks like maybe we need to implement something like the answer in this stack overflow:

Requirement.filter(params['where']).sort(customer: {name: {asc: :nulls_last}}).distinct_on(:id) =>

SELECT * FROM
(
  SELECT DISTINCT ON (requirements.id) requirements.*
  FROM "requirements"
  JOIN ...
  WHERE ...
  ORDER BY requirements.id, customers.name ASC NULLS LAST
) distinct_requirements
ORDER BY customers.name ASC NULLS LAST

Thoughts @waratuman?