SubQuery Aggregates in ActiveRecord - sql

Question

I'm trying to avoid using straight up SQL in my Rails app, but need to do a quite large version of this:
SELECT ds.product_id,
( SELECT SUM(units) FROM daily_sales WHERE (date BETWEEN '2015-01-01' AND '2015-01-08') AND service_type = 1 ) as wk1,
( SELECT SUM(units) FROM daily_sales WHERE (date BETWEEN '2015-01-09' AND '2015-01-16') AND service_type = 1 ) as wk2
FROM daily_sales as ds group by ds.product_id
I'm sure it can be done, but i'm struggling to write this as an active record statement. Can anyone help?

Answer 1

If you must do this in a single query, you'll need to write some SQL for the CASE statements. The following is what you need:
ranges = [ # ordered array of all your date-ranges
Date.new(2015, 1, 1)..Date.new(2015, 1, 8),
Date.new(2015, 1, 9)..Date.new(2015, 1, 16)
]
overall_range = (ranges.first.min)..(ranges.last.max)
grouping_sub_str = \
ranges.map.with_index do |range, i|
"WHEN (date BETWEEN '#{range.min}' AND '#{range.max}') THEN 'week#{i}'"
end.join(' ')
grouping_condition = "CASE #{grouping_sub_str} END"
grouping_columns = ['product_id', grouping_condition]
DailySale.where(date: overall_range).group(grouping_columns).sum(:units)
That will produce a hash with array keys and numeric values. A key will be of the form [product_id, 'week1'] and the value will be the corresponding sum of units for that week.

Answer 2

Simplify your SQL to the following and try converting it..
SELECT ds.product_id,
, SUM(CASE WHEN date BETWEEN '2015-01-01' AND '2015-01-08' AND service_type = 1
THEN units
END) WK1
, SUM(CASE WHEN date BETWEEN '2015-01-09' AND '2015-01-16' AND service_type = 1
THEN units
END) WK2
FROM daily_sales as ds
group by ds.product_id

Answer 3

Every rail developer sooner or later hits his/her head against the walls of Active Record query interface just to find the solution in Arel.
Arel gives you the flexibility that you need in creating your query without using loops, etc. I am not going to give runnable code rather some hints how to do it yourself:
We are going to use arel_tables to create our query. For a model called for example Product, getting the Arel table is as easy as products = Product.arel_table
Getting sum of a column is like daily_sales.project(daily_sales[:units].count).where(daily_sales[:date].gt(BEGIN_DATE).where(daily_sales[:date].lt(END_DATE). You can chain as many wheres as you want and it will be translated into SQL ANDs.
Since we need to have multiple sums in our end result you need to make use of Common Table Expressions(CTE). Take a look at docs and this answer for more info on this.
You can use those CTEs from step 3 in combination with group and you are done!