Get series of counts with conditions using ActiveRecord - sql

Question

I'm displaying a graph of Subscription counts that may have been soft_destroyed_at at some point.
To do this I run a query for each month which is of course not as good as one big honking query but my SQL skills fail me once again.
Here's how I do it in Ruby:
months = (0..12).map { |i| i.months.ago.end_of_month }
stats = Hash[months.map do |eom|
[
eom.beginning_of_month.to_date,
Subscription.where(
'created_at < ?' \
'AND (soft_destroyed_at IS NULL ' \
' OR soft_destroyed_at > ?) ' \
'AND (suspended_at IS NULL ' \
' OR suspended_at > ?)',
eom, eom, eom
).count
]
end]
# => { 2018-04-01 => 10, 2018-03-01 => 15, ... }
How could I write this as one query using ActiveRecord – or if necessary using raw SQL>
The database is Postgres 10.2, the app is Rails 5.2.
Thanks!

Answer 1

You can use this query (I used 12 month in 2017; just change as you wish). This assumes a Postgresql DB, as you said in a comment:
query =
"select months.month, count(created_at) "\
"from "\
"(select DATE '2017-01-01'+(interval '1' month * generate_series(0,11)) as month, "\
"DATE '2017-02-01'+(interval '1' month * generate_series(0,11)) as next) months "\
"outer join subscriptions on "\
"created_at < month and "\
"(soft_destroyed_at IS NULL or soft_destroyed_at >= next) and "\
"(suspended_at IS NULL OR suspended_at >= next) "\
"group by month "\
"order by month"
results = ActiveRecord::Base.connection.execute(query)
The first subquery (the select inside from) generates this:
month next
"2017-01-01 00:00:00";"2017-02-01 00:00:00"
"2017-02-01 00:00:00";"2017-03-01 00:00:00"
"2017-03-01 00:00:00";"2017-04-01 00:00:00"
"2017-04-01 00:00:00";"2017-05-01 00:00:00"
"2017-05-01 00:00:00";"2017-06-01 00:00:00"
"2017-06-01 00:00:00";"2017-07-01 00:00:00"
"2017-07-01 00:00:00";"2017-08-01 00:00:00"
"2017-08-01 00:00:00";"2017-09-01 00:00:00"
"2017-09-01 00:00:00";"2017-10-01 00:00:00"
"2017-10-01 00:00:00";"2017-11-01 00:00:00"
"2017-11-01 00:00:00";"2017-12-01 00:00:00"
"2017-12-01 00:00:00";"2018-01-01 00:00:00"
Next is only used to make it easier to check if the subscription was active at least until next month (destroyed or suspended are >= next (which guarantees the subscriber was active during current month).