Use PostgreSQL JSON result in Rails http://blog.redpanthers.co/create-json-response-using-postgresql-instead-rails/

json_serializer.rb

module JsonSerializer
  module_function

  #
  # Usage:
  # JsonSerializer.find_by_id(User, 1, only: [:id, email, :address], except: [:address])
  #
  def find_by_id(klass, id, options = {})
    sql = "SELECT row_to_json(results) FROM (
             SELECT #{selected_attributes(klass, options)}
             FROM #{table_name(klass)}
             WHERE id = #{id}
          ) as results"
    find_by_sql(sql)
  end

  #
  # Usage:
  # JsonSerializer.find_all(User, only: [:id, email, :address], except: [:address])
  #
  def find_all(klass, options = {})
    sql = "SELECT array_agg(row_to_json(results)) FROM (
             SELECT #{selected_attributes(klass, options)}
             FROM #{table_name(klass)}
          ) as results"
    find_by_sql(sql)
  end

  def find_by_sql(sql)
    result = ActiveRecord::Base.connection.execute(sql)
    return '{}' if result.ntuples == 0
    result.getvalue(0, 0)
  end

  def table_name(klass)
    klass.name.underscore.pluralize
  end

  def selected_attributes(klass, options)
    only = options[:only].present? ? options[:only].map { |a| a.to_s } : []
    exceptions = options[:except].present? ? options[:except].map { |a| a.to_s } : []
    attributes = only.empty? ? klass.column_names : only
    attributes = attributes.delete_if { |a| exceptions.include?(a) }
    attributes.join(', ')
  end
end