CODEWITHSUNDEEP
ruby-on-railsruby
airsoftFreak
airsoftFreak

Reputation: 1608

How to group_by into a given hash_map

Currently I have two models

class Author
  # gender
  # name
end

class Book
  # status -> ['published', 'in_progress']
  has_one :author

end

I decided to use group_by to group the dataset

def group_by_gender_by_status
  books.group_by { |book| [book.author.gender, book.status] }
end

What do I get instead is this

{["male", "published"] => [{BooksRecord}]
["female", "published"] => [{BooksRecord}]
["male", "in_progress"] => [{BooksRecord}]
["female", "in_progress"] => [{BooksRecord}]}

My goal is to get this result

{
  female: {
    published: 10,
    in_progress: 7
  },
  male: {
    published: 6,
    in_progress: 9
  }
}

so that I can access via data[:male][:published], easier to present the data

Upvotes: 0

Views: 72

Answers (2)

engineersmnky
engineersmnky

Reputation: 29613

Enumerable#group_by just creates keys for grouping so you cannot use this exclusively in order to produce your desired result. Additionally as books grows iterating in this fashion will be come less and less performant.

You will be better off putting the grouping and counting on the database so that return is closer to your desired end result, like so:

def group_by_gender_by_status
  books.joins(:author)
    .group(Author.arel_attribute(:gender),Book.arel_attribute(:status))
    .count
end

This will have a similar resulting Hash as your current group_by implementation however the counting and grouping will be performed on the database side before returning:

{["male", "published"] => 6,
["female", "published"] => 10,
["male", "in_progress"] => 9,
["female", "in_progress"] => 7}

To transition this into your desired nesting we will need to post process this data.

def group_by_gender_by_status
  books.joins(:author)
    .group(Author.arel_attribute(:gender),Book.arel_attribute(:status))
    .count
    .each_with_object(Hash.new {|h,k| h[k] = {}}) do |((gender,status),counter),obj| 
      obj[gender.to_sym][status.to_sym] = counter
    end 
end

The end result will be equivalent to your desired result and by moving the grouping and the counting to the database level it should degrade at a much slower rate.

Note: I have no idea where books came from or where this method currently exists. The implementation could potentially be further reduced by this understanding.

Upvotes: 1

Tom Lord
Tom Lord

Reputation: 28305

I think you can do something like this:

books.group_by { |book| book.author.gender }
  .transform_values { |books| books.map(&:status).tally }

In particular, this is leveraging Enumerable#tally, which as added to ruby version 2.7.

You didn't specify which ruby version you're actually using though, so if you're stuck on an older one, you could replace the last line with:

.transform_values { |books| books.group_by(&:status).transform_values(&:count) }

Upvotes: 1

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