ruby on rails - GroupingError: ERROR: column " " must appear in the GROUP BY clause or be used in an aggregate function

Question

I am trying to create a list of unique patients who have left comments. The code words fine until I upload to heroku where it doesnt work in postgresql.

This is my Ruby .erb code to create the list:

<% @comments.group(:patient_id).each_with_index do |comment, index| %>
    <div class="profile-post color-one">
       <span class="profile-post-numb"><%= index+1 %></span>
        <div class="profile-post-in">
            <h3 class="heading-xs"><a><%= link_to "#{comment.patient.first_name} #{comment.patient.last_name}", comment_path(comment) %></a></h3>
            <p>Lastest comment from <%= time_ago_in_words(comment.created_at) %> ago<i class="pull-right"><%= link_to "Edit", edit_comment_path(comment) %> <%= link_to "Delete", comment_path(comment), method: :delete, data: {confirm: 'Are you sure you want to delete'} %></i></p>
        </div>
    </div>
<% end %>

@comments is defined in the controller as:

def index
  @comments = current_clinician.comments.order("created_at desc")
end

heroku logs give me this as the error message:

PG::GroupingError: ERROR:  column "comments.id" must appear in the GROUP BY clause or be used in an aggregate function

LINE 1: SELECT "comments".* FROM "comments"  WHERE  comments"."clini...
               ^
SELECT "comments".* *FROM "comments"  WHERE "comments"."clinician_id" = $1 GROUP BY patient_id  ORDER BY created_at desc

I have tried the solutions from a other SO questions, such as 20942477. Which said that I should add the field comments.id to my group clause:

<% @comments.group(:patient_id, :"comments.id").each_with_index do |comment, index| %>

This gets rid of the error on heroku but defeats the purpose of the group command - it no longer only shows unique patients but instead lists all of them.

I also tried the solutions from 1780893. Which said that I should change the ORDER BY:

@comments = current_clinician.comments.order("substring(created_at,1.8) desc")

Which gives this error locally:

SQLite3::SQLException: no such function: substring: SELECT "comments".* FROM "comments" WHERE "comments"."clinician_id" = ? ORDER BY substring(created_at,1.8) desc

I realize this is a common problem, I am inexperienced with SQL so I am having trouble getting the code so it will work in both my development and production environments. The answers I have read on SO aren't using Ruby to get SQL and go over my experience level.

Answer 1

You cannot combine SELECT * with GROUP BY some_column in Postgres (unless some_column is the PK), because that's a contradiction. All non-aggregated columns (used in the SELECT, HAVING or ORDER BY clause outside an aggregate function) must be in the GROUP BY list - where the primary key column can replace all columns of a table. Else it is undefined which value to pick from the aggregated set.

Per documentation:

When GROUP BY is present, or any aggregate functions are present, it is not valid for the SELECT list expressions to refer to ungrouped columns except within aggregate functions or when the ungrouped column is functionally dependent on the grouped columns, since there would otherwise be more than one possible value to return for an ungrouped column. A functional dependency exists if the grouped columns (or a subset thereof) are the primary key of the table containing the ungrouped column.

_{A certain other RDBMS is known to play dirty tricks here and allow this and pick arbitrary values...}

You seem to want a list of unique patients that have commented, with the latest comment each. The simplest way in Postgres is with DISTINCT ON:

SELECT DISTINCT ON (patient_id) *
FROM   comments
WHERE  clinician_id = $1
ORDER  BY patient_id, created_at DESC NULLS LAST;

But this won't fly with SQLite - which should not be in the loop to begin with:

• Generic Ruby solution for SQLite3 "LIKE" or PostgreSQL "ILIKE"?

NULLS LAST is only relevant if created_at can be NULL:

• PostgreSQL sort by datetime asc, null first?

Details for DISTINCT ON:

• Select first row in each GROUP BY group?