Group by Week and Count Column Values in Rails - sql

Question

Looking to do a simple-ish SQL query in rails (using Active Record) and I'm running into some trouble.
I want to return a JSON response so the client can consume the following data structure:
{"2014-12-01-2014-12-07": {
"foo": 100,
"bar": 50,
"baz": 20,
"blah": 10,
},
"2014-12-08-2014-12-14": {
"foo": 40,
"bar": 550,
"baz": 210,
"blah": 10,
}
}
Where foo, bar, baz, blah etc. are possible values available in an array on the Foo model (from Foo::PossibleStates). I want to return summed counts of each type per week. I know roughly how I'd go about this in Mongo (the world I'm more familiar with), but am running into trouble with the nuances in SQL and Rails/Active Record. Any direction would be greatly appreciated! Here's what I've tried so far:
class FooController < ApplicationController
def index
start_date = params[:start_date]
end_date = params[:end_date]
#jsonFooData = Foo.group('created_at').group("workflow_state").sum('workflow_state')
end
end

Answer 1

I think it is not an easy task.
If you are using postgres, it could be achieved by one sql and a loop:
# models/foo.rb
# call this Foo.count_by_week in your controller
def self.count_by_week
raw_result = group_by_week_and_state.count
# {['2014-12-01-2014-12-07', 'foo'] => 100, ['2014-12-01-2014-12-07', 'bar'] => 100, '...' => '...'}
raw_result.each_with_object({}) do |(k, v), result|
result[k[0]] ||= {}
result[k[0]][k[1]] = v
end
end
def self.group_by_week_and_state
group("#{weekday_query(0)} || \'-\' || #{weekday_query(6)}").group('workflow_state')
end
# build sql part for day offset of week (0 => mon, 6 => sun)
def self.weekday_query(offset)
"to_char(cast(date_trunc(\'week\', created_at) as date) + #{offset}, \'YYYY-MM-DD\')"
end

Answer 2

results = Hash.new 0
counts = Hash.new 0
# Get the results 'foos' with all the foo records grouped by beginning of the week. I'm using 'Monday' here but can be changed to any other weekday.
foos = Foo.all.group_by{|f| f.created_at.beginning_of_week(:monday)}
# loop through each grouped item
foos.collect do |foo|
# collect the workflow states and loop through them and count the unique workflow states.
foo[1].collect{|f| f.workflow_state}.each do |ws|
counts[ws] += 1
end
# assign the workflow states counts to the beginning of the week
results[foo[0]] = counts
# reset the counter to new hash
counts = Hash.new 0
end
# output the results
puts results