Nesting loop within a block in Ruby

Question

I have a helper module to generate an array hash data, which is something like:

[{:date => d, :total_amount => 31, :first_category => 1, :second_category => 2,...},
 {:date => d+1, :total_amount => 31, :first_category => 1, :second_category => 2,...}]

So I make the method like:

def records_chart_data(category = nil, start = 3.weeks.ago)
  total_by_day = Record.total_grouped_by_day(start)
  category_sum_by_day = Record.sum_of_category_by_day(start)

  (start.to_date..Time.zone.today).map do |date|
    {
      :date  => date,
      :total_amount => total_by_day[date].try(:first).try(:total_amount) || 0,
      Category.find(1).title => category_sum_by_day[0][date].try(:first).try(:total_amount) || 0,
      Category.find(2).title => category_sum_by_day[1][date].try(:first).try(:total_amount) || 0,
      Category.find(3).title => category_sum_by_day[2][date].try(:first).try(:total_amount) || 0,
    }
  end
end

Since the Category will always change, I try to use loop in this method like:

def records_chart_data(category = nil, start = 3.weeks.ago)
  total_by_day = Record.total_grouped_by_day(start)
  category_sum_by_day = Record.sum_of_category_by_day(start)

  (start.to_date..Time.zone.today).map do |date|
    {
      :date  => date,
      Category.all.each_with_index do |category, index|             
        category.title => category_sum_by_day[index][date].try(:first).try(:total_amount) || 0,
      end
      :total_amount => total_by_day[date].try(:first).try(:total_amount) || 0
    }
  end
end

But ruby alerts me with an error:

/Users/tsu/Code/CashNotes/app/helpers/records_helper.rb:10: syntax error, unexpected tASSOC, expecting keyword_end
      category.title => category_sum_by_day[index][d...

Why does it say expecting keyword_end, and how should I fix it?

The method category_sum_by_day it calls looks like:

def self.sum_of_category_by_day(start)
  records = where(date: start.beginning_of_day..Time.zone.today)
  records = records.group('category_id, date(date)')
  records = records.select('category_id, date, sum(amount) as total_amount')
  records = records.group_by{ |r| r.category_id }
  records.map do |category_id, value|
    value.group_by {|r| r.date.to_date} 
  end
end

Or should I alter this method to generate a more friendly method for the helper above?

Jan · Accepted Answer · 2012-12-17 11:31:18Z

Category.all.each_with_index do |category, index|             
  category.title => category_sum_by_day # ...snip!
end

Unfortunately, this piece of code does not adhere to Ruby's grammar. The problem is the body of the block. x => y is not an expression and the syntax requires bodies of blocks to be expressions.

If you want to generate a hash by one key-value pair at a time try the following combination of Hash::[], Array#flatten and the splat operator (i.e. unary *):

Hash[*5.times.map { |i| [i * 3, - i * i] }.flatten]

As a result I'd rewrite the last expresion of records_chart_data more or less as follows

(start.to_date..Time.zone.today).map do |date|
  categories = Hash[*Category.all.each_with_index do |category, index|
    [ category.title, category_sum_by_day[...] ]
  end .flatten]

  { :date  => date,
    :total_amount => total_by_day[date].try(:first).try(:total_amount) || 0
  }.merge categories
end

If you consider it unreadable you can do it in a less sophisticated way, i.e.:

(start.to_date..Time.zone.today).map do |date|
  hash = {
    :date  => date,
    :total_amount => total_by_day[date].try(:first).try(:total_amount) || 0
  }
  Category.all.each_with_index do |category, index|
    hash[category.title] = category_sum_by_day[...]
  end
  hash
end

Another idea is to use Array#reduce and adopt a more functional approach.

(start.to_date..Time.zone.today).map do |date|
  Category.all.each_with_index.reduce({
    :date  => date,
    :total_amount => total_by_day[date].try(:first).try(:total_amount) || 0
  }) do |hash, (category, index)|
    hash.merge category.title => category_sum_by_day[...]
  end
  hash
end

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