gem 'activerecord-virtual-columns'

class User < ActiveRecord::Base
  include ActiveRecord::VirtualColumns
  has_many :tweets
  virtual_column :tweet_count,
    scope: -> { Tweet.where('user_id = users.id').select('COUNT(*)') },
    method: -> { tweets.count }
end

class Tweet < ActiveRecord::Base
  belongs_to :user
end

# SELECT *, (SELECT COUNT(*) FROM tweets WHERE user_id = users.id) AS tweet_count
user = User.with_vcolumns(:tweet_count).find 1
# loads from memory (does not run query)
user.tweet_count

virtual_column :uses_json,
  scope: -> { "'{ \"a\": 1 }'::json" },
  transform: ->(json_string) { JSON.load json_string }

# .maximum(:value) won't work here!
virtual_column :best_score,
  scope: -> { Score.where('user_id = users.id').select('MAX(value)') }