Idea behind #rollup method clarified

My co-worker/manager left the following comment to my previous post:

Using Enumerable methods (esp. Rail's group_by / index_by) is more idiomatic, I think:

people.group_by(&:name).each do |name, people|
puts "Details: User #{name} has residencies at: #{people.map(&:address).join('; ')"
end

eee.c

My initial code example didn't express this clearly enough but our original use case was to process thousands of studies worth of data and import them into our database. In this case we did not wish to load all of that data into memory but instead deal with each line at a time and "build up" our Study object as we went. Finally, upon starting a new study or reaching the end-of-file, we would want to save our Study to the DB.

Here is another code example that better illustrates the issue at hand:

 1 class Study < ActiveRecord::Base 
 2    # assume fields like: external_id, title 
 3    # also a collection of study_team members for the following roles: 
 4    #      'Billing Coordinator' & 'Admin Assistant' 
 5 end 
 6  
 7 class StudyProcessor 
 8   # process csv files in the following format: 
 9   #    external_id, title, billing_coord, admin_asst. 
10   #    1, "Oncology Study 1", "Emerson, Ralph", "Sanders, Colonel" 
11   #    1, "Oncology Study 1",, "Roberts, Julia" 
12   #    1, "Oncology Study 1", "Manners, Miss",  
13   #    2, "Test Drug Study 2", "Pusher, Imyour", "Spears, Courtenay" 
14   #    .... On for 20000 rows ... 
15   #     
16   def initialize(file) 
17     @file = file # CSV file with thousands of rows 
18   end 
19  
20   def process 
21     csv_reader = CSV.open('csvfile.csv', 'r')  
22     # add #rollup method to our reader 
23     class << csv_reader; include Patterns; end; 
24      
25     # let us assume that we can either pass a proc or a symbol representing a locally scoped method to rollup
26     # Also it is a prereq that the study data rows already be grouped together! 
27     csv_reader.rollup( :study_data_csv_key, :initialize_new_study, :merge_study_team_into_study, :save_study ) 
28   ensure 
29     csv_reader.close rescue nil 
30   end 
31  
32  
33  
34   def initialize_new_study(data_line) 
35     @study = Study.new(data_line[0], data_line[1])   
36   end 
37  
38   def merge_study_team_data_into_study(data_line) 
39     unless @study.billing_coordinators.include?(data_line[2]) 
40       @study.billing_coordinators << data_line[2]) 
41     end 
42     unless @study.admin_assistants.include?(data_line[3]) 
43       @study.admin_assistants << data_line[3]) 
44     end 
45   end 
46  
47   def study_data_csv_key(data_line) 
48     data_line[0]  # let us use external id  
49   end 
50  
51   def save_study 
52     @study.save! 
53   end 
54  
55 end