class Unit < ApplicationRecord
  ATTRIBUTES = [:symbol, :description, :multiplier, :base_id]

  belongs_to :user, optional: true
  belongs_to :base, optional: true, class_name: "Unit"
  has_many :subunits, class_name: "Unit", dependent: :restrict_with_error, inverse_of: :base

  validate if: ->{ base.present? } do
    errors.add(:base, :user_mismatch) unless user == base.user
    errors.add(:base, :multilevel_nesting) if base.base.present?
  end
  validates :symbol, presence: true, uniqueness: {scope: :user_id},
    length: {maximum: columns_hash['symbol'].limit}
  validates :description, length: {maximum: columns_hash['description'].limit}
  validates :multiplier, numericality: {equal_to: 1}, unless: :base
  validates :multiplier, numericality: {other_than: 0}, if: :base

  scope :defaults, ->{ where(user: nil) }
  scope :defaults_diff, ->{
    actionable_units = Arel::Table.new('actionable_units')
    units = actionable_units.alias('units')
    bases_units = arel_table.alias('bases_units')
    other_units = arel_table.alias('other_units')
    other_bases_units = arel_table.alias('other_bases_units')
    sub_units = arel_table.alias('sub_units')

    Unit.with_recursive(actionable_units: [
      Unit.with(units: self.or(Unit.defaults)).left_joins(:base)
        .where.not(
          # Exclude Units that are/have default counterpart
          Arel::SelectManager.new.project(1).from(other_units)
            .outer_join(other_bases_units)
            .on(other_units[:base_id].eq(other_bases_units[:id]))
            .where(
              other_bases_units[:symbol].is_not_distinct_from(bases_units[:symbol])
                .and(other_units[:symbol].eq(arel_table[:symbol]))
                .and(other_units[:user_id].is_distinct_from(arel_table[:user_id]))
            ).exists
        )
        .select(
          arel_table[Arel.star],
          # Decide if Unit can be im-/exported based on existing hierarchy:
          # * same base unit symbol has to exist
          # * unit with subunits can only be ported to root
          arel_table[:base_id].eq(nil).or(
            (
              Arel::SelectManager.new.project(1).from(other_units)
                .join(sub_units).on(other_units[:id].eq(sub_units[:base_id]))
                .where(
                  other_units[:symbol].eq(arel_table[:symbol])
                    .and(other_units[:user_id].is_distinct_from(arel_table[:user_id]))
                ).exists.not
            ).and(
              Arel::SelectManager.new.project(1).from(other_bases_units)
                .where(
                  other_bases_units[:symbol].is_not_distinct_from(bases_units[:symbol])
                    .and(other_bases_units[:user_id].is_distinct_from(bases_units[:user_id]))
                ).exists
            )
          ).as('portable')
        ),
      # NOTE: turn off ONLY_FULL_GROUP_BY or is it incompatible with other DBs?
      # Fill base Units to display proper hierarchy. Duplicates will be removed
      # by final group() - can't be deduplicated with UNION due to 'portable' field.
      arel_table.join(actionable_units).on(actionable_units[:base_id].eq(arel_table[:id]))
        .project(arel_table[Arel.star], Arel::Nodes.build_quoted(nil).as('portable'))
    ]).select(units: column_names).select(units[:portable].minimum.as('portable'))
      .from(units).group(Unit.column_names)
  }
  scope :ordered, ->{
    left_outer_joins(:base)
      .order(arel_table.coalesce(Arel::Table.new(:bases_units)[:symbol], arel_table[:symbol]),
             arel_table[:base_id].asc.nulls_first, :multiplier, :symbol)
  }

  before_destroy do
    # TODO: disallow destruction if any object depends on this unit
    nil
  end

  def to_s
    symbol
  end

  def movable?
    subunits.empty?
  end

  def default?
    user_id.nil?
  end

  def exportable?
    !default? && (base.nil? || base.default?)
  end
end