fixin.me/app/models/unit.rb

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", inverse_of: :base,
    dependent: :restrict_with_error

  validate if: ->{ base.present? } do
    errors.add(:base, :user_mismatch) unless user_id == base.user_id
    errors.add(:base, :self_reference) if id == base_id
    errors.add(:base, :multilevel_nesting) if base.base_id?
  end
  validates :symbol, presence: true, uniqueness: {scope: :user_id},
    length: {maximum: type_for_attribute(:symbol).limit || Float::INFINITY}
  validates :description, length: {maximum: type_for_attribute(:description).limit || Float::INFINITY}
  validates :multiplier, numericality: {equal_to: 1}, unless: :base
  validates :multiplier, numericality: {greater_than: 0, precision: true, scale: true}, 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: [
      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')
        ),
      # Fill base Units to display proper hierarchy. Duplicates will be removed
      # by final group() - can't be deduplicated with UNION due to 'portable' field.
      # Use ActiveRecord::Relation (not a raw SelectManager) so the SQLite Arel
      # visitor does not wrap it in parentheses inside the UNION ALL CTE body.
      Unit.joins(
        arel_table.create_join(
          actionable_units,
          arel_table.create_on(actionable_units[:base_id].eq(arel_table[:id]))
        )
      ).select(arel_table[Arel.star], Arel::Nodes.build_quoted(nil).as('portable'))
    ]).select(units: [:base_id, :symbol])
      .select(
        units[:id].minimum.as('id'), # can be ANY_VALUE()
        units[:user_id].minimum.as('user_id'), # prefer non-default
        Arel::Nodes.build_quoted(1).as('multiplier'), # disregard multiplier when sorting
        units[:portable].minimum.as('portable')
      )
      .from(units).group(units[:base_id], units[:symbol])
  }
  scope :ordered, ->{
    left_outer_joins(:base).order(ordering)
  }

  def self.ordering
    [arel_table.coalesce(Arel::Table.new(:bases_units)[:symbol], arel_table[:symbol]),
     arel_table[:base_id].not_eq(nil),
     arel_table[:multiplier],
     arel_table[:symbol]]
  end

  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

  # Should only by invoked on Units returned from #defaults_diff which are #portable
  def port!(recipient)
    recipient_base = base && Unit.find_by!(symbol: base.symbol, user: recipient)
    params = slice(ATTRIBUTES - [:symbol, :base_id])
    Unit.find_or_initialize_by(user: recipient, symbol: symbol)
      .update!(base: recipient_base, **params)
  end

  def successive
    units = Unit.arel_table
    lead = Arel::Nodes::NamedFunction.new('LAG', [units[:id]])
    window = Arel::Nodes::Window.new.order(*Unit.ordering)
    lag_id = lead.over(window).as('lag_id')
    Unit.with(
      units: user.units.left_outer_joins(:base).select(units[Arel.star], lag_id)
    ).where(units[:lag_id].eq(id)).first
  end
end