Select unique records and display as category headers in rails - sql

Question

I have a rails 3.2 app running on PostgreSQL, and have some data I want to display in my view, which is stored in the database in this structure:
+----+--------+------------------+--------------------+
| id | name | sched_start_date | task |
+----+--------+------------------+--------------------+
| 1 | "Ben" | 2013-03-01 | "Check for debris" |
+----+--------+------------------+--------------------+
| 2 | "Toby" | 2013-03-02 | "Carry out Y1.1" |
+----+--------+------------------+--------------------+
| 3 | "Toby" | 2013-03-03 | "Check oil seals" |
+----+--------+------------------+--------------------+
I would like to display a list of tasks for each name, and for the names to be ordered ASC by the first sched_start_date they have, which should look like ...
Ben
2013-03-01 – Check for debris
Toby
2013-03-02 – Carry out Y1.1
2013-03-03 – Check oil seals
The approach I starting taking was to run a query for unique names and order them by sched_start_date ASC, then run a query for each name to get their tasks.
To get a list of unique names, the SQL would look like this.
select *
from (
select distinct on (name) name, sched_start_date
from tasks
) p
order by sched_start_date;
I would like to know if this is the correct approach (querying for unique names then running another query for all their tasks), or if there is a better rails way.

Answer 1

To get the data sorted like you describe, you might want to use min() as window function in the ORDER BY clause:
SELECT name, sched_start_date, task
FROM tasks
ORDER BY min(sched_start_date) OVER (PARTITION BY name), 1, 2, 3
Your original query would need an additional ORDER BY item to get the earliest date per name:
SELECT DISTINCT ON (name) name, sched_start_date, task
FROM tasks
ORDER BY 1, 2, 3;
I also added task (3) as last ORDER BY item to break ties, in case there can be more than one per date.
But the output is still ordered by name, not by date.
Getting your peculiar format with all data stuffed into one column is a bit more complex:
SELECT one_col
FROM (
WITH x AS (
SELECT name, min(sched_start_date) AS min_start
FROM tasks
GROUP BY 1
)
SELECT 2 AS rnk, name
,sched_start_date::text || ' – ' || task AS one_col
,sched_start_date, min_start
FROM tasks
JOIN x USING (name)
UNION ALL
SELECT 1 AS rnk, name, name, NULL::date, min_start
FROM x
ORDER BY min_start, name, rnk, sched_start_date, task
) y

Answer 2

Assuming that you have associations in your model you would be able to run
#employees = Employee.order(:name, :sched_start_date, :task).includes(:tasks)
You could then iterate over them:
#employees.each do |employee|
employee.name
employee.tasks.each do |task|
task.name
end
end
This isn't gonna exactly match your needs, but should show you where to start.