class SiteStatistic < ActiveRecord::Base
  # prevents the creation of multiple rows
  default_value_for :id, 1

  COUNTER_ATTRIBUTES = %w(repositories_count wikis_count).freeze
  REQUIRED_SCHEMA_VERSION = 20180629153018

  # Tracks specific attribute
  #
  # @param [String] raw_attribute must be one of the values listed in COUNTER_ATTRIBUTES
  def self.track(raw_attribute)
    with_statistics_available(raw_attribute) do |attribute|
      SiteStatistic.update_all(["#{attribute} = #{attribute}+1"])
    end
  end

  # Untracks specific attribute
  #
  # @param [String] raw_attribute must be one of the values listed in COUNTER_ATTRIBUTES
  def self.untrack(raw_attribute)
    with_statistics_available(raw_attribute) do |attribute|
      SiteStatistic.update_all(["#{attribute} = #{attribute}-1 WHERE #{attribute} > 0"])
    end
  end

  # Wrapper for track/untrack operations with basic validations and enforced requirements
  #
  # @param [String] raw_attribute must be one of the values listed in COUNTER_ATTRIBUTES
  # @yield [String] attribute quoted to be used inside SQL / Arel query
  def self.with_statistics_available(raw_attribute)
    unless raw_attribute.in?(COUNTER_ATTRIBUTES)
      raise ArgumentError, "Invalid attribute: '#{raw_attribute}' to '#{caller_locations(1, 1)[0].label}' method. " \
                           "Valid attributes are: #{COUNTER_ATTRIBUTES.join(', ')}"
    end

    return unless available?

    self.fetch # make sure record exists

    attribute = self.connection.quote_column_name(raw_attribute)

    # will be running on its own transaction context
    yield(attribute)
  end

  # Returns a site statistic record with tracked information
  #
  # @return [SiteStatistic] record with tracked information
  def self.fetch
    SiteStatistic.transaction(requires_new: true) do
      SiteStatistic.first_or_create!
    end
  rescue ActiveRecord::RecordNotUnique
    retry
  end

  # Return whether required schema change is available
  #
  # This is needed in order to degrade gracefully when testing schema migrations
  #
  # @return [Boolean] whether schema is available
  def self.available?
    @available_flag ||= ActiveRecord::Migrator.current_version >= REQUIRED_SCHEMA_VERSION
  end

  # Resets cached column information
  #
  # This is called during schema migration specs, in order to reset internal cache state
  def self.reset_column_information
    @available_flag = nil

    super
  end
end