passive_columns

Passive Columns

A gem that extends Active Record to retrieve columns from DB on demand.
Works with Rails >= 6.1 and Ruby >= 2.7

Usage

  class Page < ApplicationRecord
    include PassiveColumns
    passive_columns :huge_article
  end

ActiveRecord::Relation now retrieves all the columns except the passive ones by default.

  article = Page.where(status: :active).to_a
  # => SELECT "pages"."id", "pages"."status", "pages"."title" FROM "pages" WHERE "pages"."status" = 'active'

If you specify the columns via select it retrieves only the specified columns and nothing more.

  page = Page.select(:id, :title).take # => #<Page id: 1, title: "Some title">
  page.to_json # => {"id": 1, "title": "Some title"}

But you still has an ability to retrieve the passive column on demand

  page.huge_article
  # => SELECT "pages"."huge_article" WHERE "pages"."id" = 1 LIMIT 1
  'Some huge article...'

  page.to_json # => {"id": 1, "title": "Some title", "huge_article": "Some huge article..."}

  # The next time you call the passive column it won't hit the database as it is already loaded.
  page.huge_article # => 'Some huge article...'

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Another way to get columns on demand is to use the load_column method.

This method loads a column value, if not already loaded, from the database regardless of whether the column is added to passive_columns or not.

  class User < ActiveRecord::Base
    include PassiveColumns
  end

user = User.select('id').take!
user.name # missing attribute 'name' for User (ActiveModel::MissingAttributeError)

user.load_column(:name) # => SELECT "name" FROM "users" WHERE "id" = ? LIMIT ?
'John'
user.load_column(:name) # no additional query. It's already loaded
'John'

user.name
'John'

By the way, it uses the Rails' .pick method to get the value of the column under the hood

Important

If you want passive_columns to skip validation rules specific to the columns you exclude.
(in case they were not retrieved / modified)

validates :huge_article, presence: true
# Will be transformed into:
# validates :huge_article, presence: true, if: -> { attributes.key?('huge_article') }

You must declare validation rules for passive_columns separately

class Page < ActiveRecord::Base
  include PassiveColumns
  passive_columns :huge_article # Declare columns above the validation rules.

  validates :name, presence: true
  # Validation rules transformation will work
  validates :huge_article, presence: true # It works for a separate rule.
  # -> the rule is transformed into:
  # -> validates :huge_article, presence: true, if: -> { attributes.key?('huge_article') }
end

class Page < ActiveRecord::Base
  include PassiveColumns
  passive_columns :huge_article # Declare columns above the validation rules.

  # Validation rules transformation WON'T work
  validates :name, :huge_article, presence: true # It doesn't work for combined rules.
  # -> the rule remains the same:
  # -> validates :name, :huge_article, presence: true
end

Installation

Add this line to your Gemfile:

gem "passive_columns"

And then execute:

$ bundle install

Or install it yourself as:

$ gem install passive_columns

Motivation

There are situations when you have an Active Record model with columns that you don't want to fetch from a DB every time you manipulate the model.

What options do you have?

# You can declare a scope to exclude columns dynamically from the select settings.
scope :skip_retrieving, ->(*v) { select(column_names.map(&:to_sym) - Array.wrap(v)) }
# or you can select only the columns you need
scope :only_main_columns, -> { select(%w[id name description uuid]) }

# When it's really important to skip unnecessary columns, you can use the default scope.
default_scope { :only_main_columns }

At first glance, it seems like a good solution. Until you realize that you cannot manipulate the model without the columns you skipped, as there are validation rules related to them.

class Project < ActiveRecord::Base
  scope :only_main_columns, -> { select(%w[id name description uuid]) }

  validates :id, :name, presence: true
  validates :settings, presence: true
end


p = Project.only_required_columns.take
p.update!(name: 'New name') # missing attribute 'settings' for Project (ActiveModel::MissingAttributeError)

One way to avoid this is to check for the presence of the attribute before validating it.

validates :huge_article, presence: true, if: -> { attributes.key?('huge_article') }

Unfortunately, boilerplate code is needed for such a simple task.
But the only thing you wanted was to exclude some columns and be able to manipulate a model without extra steps.

By the way, after doing those steps, you still cannot retrieve the column when you need it after loading the scoped model...

So, passive_columns tries to solve this problem by allowing you to exclude columns from the selection and also allowing you to retrieve them on demand when needed.

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Inspiration

There are similar gems that were relatively popular but are no longer supported. Let's give them the honor they deserve:

• lazy_columns
• columns_on_demand

License

The gem is available as open source under the terms of the MIT License.