How to group rows based on multiple fields? - ruby-on-rails

Question

I have a notifications table where I email out reports to people based on the frequency they've selected.
If an email address exists across multiple questions and has the same frequency...then I need to group them together so I can send one report for both questions.
[
#<Notification id: 1, question_id: 58, email: "john#example.com", frequency: "daily">,
#<Notification id: 2, question_id: 25, email: "john#example.com", frequency: "daily">,
#<Notification id: 3, question_id: 47, email: "john#example.com", frequency: "monthly">,
#<Notification id: 3, question_id: 19, email: "frank#example.org", frequency: "monthly">
]
So in my example data, 3 reports would be sent:
1 to john#example.com for question's 58 and 25 on a daily basis
1 to john#example.com for question 47 on a monthly basis
1 to frank#example.org for question 19 on a monthly basis
I may not be explaining this very well, so let me know if something needs clarification.

Answer 1

You can achieve this with a regular group_by:
#notifications = your_method_to_retrieve_notifications
#notifications = #noticications.group_by do |notif|
notif.ferquency + ':' + notif.email
end
This will group your notifications like this:
#notifications = {
'daily:john#example.com' => [<Notification id: 1>, #etc.],
'monthly:john#example.com' => [# list of notifications],
'monthly:frank#example.org' => [# list of notif]
}
If you want only an array of list of notifications grouped by frequency & email, use the above method and add this:
#notifications = your_method_to_retrieve_notifications
#notifications = #noticications.group_by do |notif|
notif.ferquency + ':' + notif.email
end.values
# returns Array of arrays like:
[
[<Notification id: 1 frequency: "daily", email "a#b.com">,<Notification id: 2 frequency: "daily", email "a#b.com">],
[<Notification id: 3 frequency: "daily", email "xx#yy.com">],
[<Notification id: 4 frequency: "monthly", email "a#b.com">,<Notification id: 5 frequency: "monthly", email "a#b.com">],
]