Question

Using

Ruby  1.9.3-p194  
Rails 3.2.8

Here’s what I need.
Count the different human resources (human_resource_id) and divide this by the total number of assignments (assignment_id).
So, the answer for the dummy-data as given below should be:

1.5 assignments per human resource

But I just don’t know where to go anymore.
Here’s what I tried:

Table name: Assignments

id       | human_resource_id | assignment_id | assignment_start_date | assignment_expected_end_date
80101780 | 20200132          | 80101780      | 2012-10-25            | 2012-10-31
80101300 | 20200132          | 80101300      | 2012-07-07            | 2012-07-31
80101308 | 21100066          | 80101308      | 2012-07-09            | 2012-07-17

At first I need to make a selection for the period I need to ‘look’ at. This is always from max a year ago.

a = Assignment.find(:all, :conditions => { :assignment_expected_end_date => (DateTime.now - 1.year)..DateTimenow })
=> [
#<Assignment id: 80101780, human_resource_id: "20200132", assignment_id: "80101780", assignment_start_date: "2012-10-25", assignment_expected_end_date: "2012-10-31">, 
#<Assignment id: 80101300, human_resource_id: "20200132", assignment_id: "80101300", assignment_start_date: "2012-07-07", assignment_expected_end_date: "2012-07-31">, 
#<Assignment id: 80101308, human_resource_id: "21100066", assignment_id: "80101308", assignment_start_date: "2012-07-09", assignment_expected_end_date: "2012-07-17">
]

foo = a.group_by(&:human_resource_id)

Now I got a beautiful ‘Array of hash of object’ and I just don’t know what to do next.

Can someone help me?

Answer 1

You can try to execute the request in SQL :

ActiveRecord::Base.connection.select_value('SELECT count(distinct human_resource_id) / count(distinct assignment_id) AS ratio FROM assignments');