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Eito Katagiri edited this page Nov 14, 2017 · 1 revision

Comma is a CSV (i.e. comma separated values) generation extension for Ruby objects, that lets you seamlessly define a CSV output format via a small DSL. Comma works well on pure Ruby objects with attributes, as well as complex ones such as ActiveRecord objects with associations, extensions, etc. It doesn't distinguish between attributes, methods, associations, extensions, etc. - they all are considered equal and invoked identically via the Comma DSL description. Multiple different CSV output descriptions can also be defined.

When multiple objects in an Array are converted to CSV, the output includes generation of a header row reflected from names of the properties requested, or specified via the DSL.

CSV can be a bit of a boring format - the motivation behind Comma was to have a CSV extension that was simple, flexible, and would treat attributes, methods, associations, etc., all the same without the need for any complex configuration, and also work on Ruby objects, not just ActiveRecord or other base class derivatives.

An example Comma CSV enabled ActiveRecord class:

class Book < ApplicationRecord
  # ================
  # = Associations =
  # ================
  has_many   :pages
  has_one    :isbn
  belongs_to :publisher

  # ===============
  # = CSV support =
  # ===============
  comma do
    name
    description

    pages :size => 'Pages'
    publisher :name
    isbn number_10: 'ISBN-10', number_13: 'ISBN-13'
    blurb 'Summary'
  end
end

Annotated, the comma description is as follows:

# starts a Comma description block, generating 2 methods #to_comma and
# #to_comma_headers for this class.
comma do
  # name, description are attributes of Book with the header being reflected as
  # 'Name', 'Description'
  name
  description

  # pages is an association returning an array, :size is called on the
  # association results, with the header name specified as 'Pages'
  pages size: 'Pages'

  # publisher is an association returning an object, :name is called on the
  # associated object, with the reflected header 'Name'
  publisher :name

  # isbn is an association returning an object, :number_10 and :number_13 are
  # called on the object with the specified headers 'ISBN-10' and 'ISBN-13'
  isbn :number_10 => 'ISBN-10', :number_13 => 'ISBN-13'

  # blurb is an attribute of Book, with the header being specified directly
  # as 'Summary'
  blurb 'Summary'
end

In the above example, any of the declarations (name, description, pages, publisher, isbn, blurb, etc), could be methods, attributes, associations, etc - no distinction during configuration is required, as everything is invoked via Ruby's #send method.

You can get the CSV representation of any object by calling the to_comma method, optionally providing a CSV description name to use.

Object values are automatically converted to strings via to_s allowing you to reuse any existing to_s methods on your objects (instead of having to call particular properties or define CSV specific output methods). Header names are also automatically humanised when reflected (eg. Replacing _ characters with whitespace). The 'isbn' example above shows how multiple values can be added to the CSV output.

Multiple CSV descriptions can also be specified for the same class, eg:

class Book < ActiveRecord::Base

  # ================
  # = Associations =
  # ================
  has_many   :pages
  has_one    :isbn
  belongs_to :publisher

  # ===============
  # = CSV support =
  # ===============
  comma do
    name
    description

    pages size: 'Pages'
    publisher :name
    isbn number_10: 'ISBN-10', number_13: 'ISBN-13'
    blurb 'Summary'
  end

  comma :brief do
    name
    description
    blurb 'Summary'
  end
end

You can specify which output format you would like to use via an optional parameter to to_comma:

Book.limit(10).to_comma(:brief)

Specifying no description name to to_comma is equivalent to specifying :default as the description name.

You can pass all options for the CSV renderer (See: CSV.new), e.g.

Book.limit(10).to_comma(style: :brief, col_sep: ';', force_quotes: true)

You can pass the :filename option and have Comma writes the CSV output to this file:

Book.limit(10).to_comma(filename: 'books.csv')

You also can pass the :write_header option to hide the header line (true is default):

Book.limit(10).to_comma(write_headers: false)

##Using blocks

For more complex relationships you can pass blocks for calculated values, or related values. Following the previous example here is a comma set using blocks (both with and without labels for your CSV headings):

class Publisher < ApplicationRecord
  # ================
  # = Associations =
  # ================
  has_one :primary_contact, class_name: 'User' #(basic user with a name)
  has_many :users
end

class Book < ApplicationRecord
  # ================
  # = Associations =
  # ================
  has_many   :pages
  has_one    :isbn
  belongs_to :publisher

  # ===============
  # = CSV support =
  # ===============
  comma do
    name
    description

    pages :size => 'Pages'
    publisher :name
    publisher { |publisher| publisher.primary_contact.name.to_s.titleize }
    publisher 'Number of publisher users' do |publisher| publisher.users.size end
    isbn :number_10 => 'ISBN-10', :number_13 => 'ISBN-13'
    blurb 'Summary'
  end
end

In the preceding example, the 2 new fields added (both based on the publisher relationship) mean that the following will be added:

  • the first example 'publishers_contact' is loaded straight as a block. The value returned by the lambda is displayed with a header value of 'Publisher'
  • the second example 'total_publishers_users' is sent via a hash and a custom label is set, if used in the first examples method the header would be 'Publisher', but sent as a hash the header is 'Number of publisher users'.

Using special fields

__use__

With __use__ field, you can reuse output formats that are defined in the same class. In the example below, default format (:default) includes :minimum format when #to_comma is called.

class Book < ApplicationRecord
  comma do
    __use__ :minimum

    description

    isbn number_10: 'ISBN-10', number_13: 'ISBN-13'
  end

  comma :minimum do
    name
  end
end

__static_column__

When you want to have static value in your CSV output, you can use __static__ field. You can provide values to output to blocks. Without block, field will become empty.

class Book < ActiveRecord::Base
  comma do
    __static_column__ 'Check' do ' ' end
    name
    __static_column__ 'Spacer'
    description
  end
end

Using With Ruby on Rails

When used with Rails (ie. add 'comma' as a gem dependency), Comma automatically adds support for rendering CSV output in your controllers:

class BooksController < ApplicationController
  def index
    respond_to do |format|
      format.csv { render csv: Book.limit(50) }
    end
  end
end

You can specify which output format you would like to use by specifying a style parameter or adding any available CSV option:

class BooksController < ApplicationController
  def index
    respond_to do |format|
      format.csv { render csv: Book.limit(50), style: :brief }
    end
  end
end

You can also specify a different file extension ('csv' by default)

class BooksController < ApplicationController
  def index
    respond_to do |format|
      format.csv { render csv: Book.limit(50), extension: 'txt' }
    end
  end
end

With the Rails renderer you can supply any of the regular parameters that you would use with to_comma such as :filename, :write_headers, :force_quotes, etc. The parameters just need to be supplied after you specify the collection for the csv as demonstrated above.

When used with Rails, comma also adds support for exporting scopes:

class Book < ActiveRecord::Base
  scope :recent, => { { conditions: ['created_at > ?', 1.month.ago] } }
  # ...
end

Calling the to_comma method on the scope will internally use Rails' find_each method, instantiating only 1,000 ActiveRecord objects at a time:

Book.recent.to_comma
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