Rails中的“复杂”分组和索引编制？

Question

I have a problem to which I can't seem to find a simple solution.

I want to achieve the following:
* i have a list of tasks, each with an owner and a due date
* i want to display a list of all tasks grouped by owner
* i want to sort the owners based on the due dates: eg The owner with the lowest due date first, followed by the owner with the second lowest, etc
* I want to be able to paginate the results, preferable with will_paginate

To ilustrate, this would be a result i am looking for:

Harry
- task 1, due date 1
- task 3, due date 4
Ben
- task 2, due date 2
Carol
- task 4, due date 3

So far, I can query for all owners with tasks, sort them on a virtual attribute with their "earliest due date" and then display them and their tasks in a view.

There are multiple problems with this approach, imo:
* i run multiple queries from the view (owner.tasks.each etc). I always learned that running queries from the view is bad
* i'm doing the sorting by loading all records in memory (virtual attribute), which could get problematic with large records set. * I have no idea how i can get pagination in on this, that will be sensitive to the amount of tasks displayed

I can't seem to crack this nut, so any help or hints would be greatly appreciated!
Erwin

Answer 1

Try this query, you have not provided sample data (ideally using SQL) so that we could play ourselves:

SELECT
u.id as owner_id, u.name as owner_name, t.id, t.due_date
FROM users u
INNER JOIN tasks m ON u.id = m.owner_id
INNER JOIN tasks t ON u.id = t.owner_id
GROUP BY u.id, u.name, t.id, t.due_date
ORDER BY MIN(m.due_date), t.due_date

You should get all the data you need in the proper order, and you can paginate simply by applying LIMIT to it (or converting it to AR and submitting it to will_paginate).