How can I avoid running singular expressions against arrays in activerecord and rails 3?

Question

I am sorry if I am asking the question poorly. I have a Rails 3.1 app with models (simplified) like so:

class Employee < ActiveRecord::Base
  has_many    :merged_children, :class_name => 'Employee', :foreign_key => "merge_parent_id"
  has_many    :timesheets

  def total_time 
    merged_children.timesheets.in_range(range).hours_minutes.sum 
  end
end

class Timesheet < ActiveRecord::Base
  belongs_to :employee

  def in_range(range)
    # filter records based on transaction_date in range 
  end

  def hours_minutes
    (hours + minutes/60.0).to_f
  end
end

Note: The in_range method acts as a scope, essentially, and hours_minutes is a calculation. hours_minutes is valid for each timesheet record in the resulting dataset, and then total_time should sum those values and return the amount.

The "total_time" method is not working because employee.merged_children returns an array and timesheets is meant to run against a single Employee object.

Is there any way to structure the "total_time" so that it still sends one query to the db? It seems inelegant to iterate over the merged_children array, issuing a query for each. Not sure if a direct call to an Arel table would help or hurt, but I am open to ideas.

If we get it right, the resulting SQL should effectively look something like:

SELECT sum(hours + minutes/60.0)
FROM employees e1 join employees e2 on e1.id = e2.merge_parent_id join timesheets t on t.employee_id = e2.id
WHERE e1.id = [@employee.id] and t.transaction_date BETWEEN [@range.begin] and [@range.end]

Thanks so much!

Answer 1

The easiest thing here might be to add

has_many :children_timesheets, :through => :merged_children, :source => :timesheets

To your employee model,

Then (assuming in_range is actually a scope, or a class method that does a find)

children_timesheets.in_range(...)

Should be the collection of timesheets you're interested in and you can do something like

children_timesheets.in_range(...).collect(&:hours_minutes).sum

Answer 2

Untested with actual data.

  range = ((1.day.ago)...(2.days.ago))
  merge_parent = Employee.find(some_id)

  Timesheet.where(:transaction_date => range)
     .joins(:employee).where(:employees => {:merge_parent_id => merge_parent.id})
     .sum('hours*60 + minutes')

   (0.3ms)  SELECT SUM(hours*60 + minutes) AS sum_id FROM "timesheets" INNER JOIN "employees" ON "employees"."id" = "timesheets"."employee_id" WHERE "employees"."merge_parent_id" = 1 AND ("timesheets"."created_at" >= '2011-12-13 03:04:35.085416' AND "timesheets"."created_at" < '2011-12-12 03:04:

Returns "0" for me. So hopefully it will return something nicer for you