sure/app/models/recurring_transaction/declared_bill.rb

class RecurringTransaction
  # The one declared-bill build path, shared by the add-bill form and the AI
  # create tool so the two can never drift on the rules that matter here:
  # the account must be one the user can actually write (attaching a bill
  # changes what that account's owners see, so a read-only share is not a
  # destination), and the amount carries the sign convention (income is
  # stored negative).
  class DeclaredBill
    attr_reader :family, :user, :attrs

    def initialize(family:, user:, attrs:)
      @family = family
      @user = user
      @attrs = attrs
    end

    def build
      account = Account.writable_by(user).find_by(id: attrs[:account_id])
      due = begin
        Date.parse(attrs[:first_due_on].to_s)
      rescue Date::Error
        nil
      end

      is_income = ActiveModel::Type::Boolean.new.cast(attrs[:is_income]) || false
      # BigDecimal parses "Infinity" and "NaN" as non-finite numbers; both are
      # invalid here, same as unparseable input.
      amount = begin
        BigDecimal(attrs[:amount].to_s.presence || "0").abs
      rescue ArgumentError
        nil
      end
      # update_bill rejects zero outright, and the schema says minimum 0.01, but
      # strict mode is off so nothing enforced it here: a zero bill was written
      # happily, and its cycles can never close because close_worthy? asks for a
      # positive expected amount, so it sat open and overdue forever.
      amount = nil if amount && (!amount.finite? || amount.zero?)
      amount = -amount if amount && is_income

      recurring = family.recurring_transactions.new(
        name: attrs[:name],
        amount: amount,
        bill_type: is_income ? "income" : "bill",
        account: account,
        currency: account&.currency || family.currency,
        payment_url: attrs[:payment_url],
        autopay: ActiveModel::Type::Boolean.new.cast(attrs[:autopay]) || false,
        notes: attrs[:notes],
        status: "active",
        manual: true,
        occurrence_count: 0
      )
      recurring.frequency_preset = attrs[:frequency_preset]
      recurring.first_due_on = attrs[:first_due_on]

      # A chosen account that does not resolve to something writable is said
      # out loud, not silently dropped: a read-only share or a foreign id
      # would otherwise become an accountless bill in the family currency.
      if attrs[:account_id].present? && account.nil?
        recurring.errors.add(:base, I18n.t("recurring_transactions.create.account_invalid"))
        return recurring
      end

      if amount.nil?
        recurring.errors.add(:base, I18n.t("recurring_transactions.create.amount_invalid"))
        return recurring
      end

      if due.nil?
        recurring.errors.add(:base, I18n.t("recurring_transactions.create.due_date_required"))
        return recurring
      end

      recurring.expected_day_of_month = due.day
      recurring.anchor_date = due
      recurring.last_occurrence_date = due
      recurring.next_expected_date = due

      FrequencyPreset.apply(
        recurring,
        preset: attrs[:frequency_preset],
        day_of_month: due.day,
        weekday: due.wday,
        month_of_year: due.month
      )

      recurring
    end

    # A series for this identifier may already exist; a second legitimate one
    # (another tier from the same biller) is distinguished by its amount, so
    # the amount is stamped into dedup_scope before the first insert. The same
    # identity at the same amount then collides immediately and is reported as
    # a validation error rather than an escaping exception.
    def self.save(recurring)
      recurring.dedup_scope = recurring.amount.to_d.to_s("F") if recurring.dedup_scope.blank?

      # Every dedup index is keyed on account_id, and Postgres treats NULLs as
      # distinct, so a bill created without an account collides with nothing.
      # Declaring one is explicitly supported ("omit it to create the bill
      # without an account"), which meant a retried create silently doubled the
      # family's recurring commitments. No partial index can express this, so
      # the check lives here.
      if recurring.account_id.nil? && account_less_duplicate?(recurring)
        recurring.errors.add(:base, I18n.t("recurring_transactions.create.already_exists"))
        return false
      end

      recurring.save
    rescue ActiveRecord::RecordNotUnique
      recurring.errors.add(:base, I18n.t("recurring_transactions.create.already_exists"))
      false
    end

    # Mirrors the identity the unique indexes use, minus the account that is
    # absent by definition: a merchant-keyed row matches on merchant, a
    # name-keyed row matches on name.
    def self.account_less_duplicate?(recurring)
      scope = recurring.family.recurring_transactions
                       .where(account_id: nil,
                              currency: recurring.currency,
                              dedup_scope: recurring.dedup_scope,
                              destination_account_id: recurring.destination_account_id)
      scope = scope.where.not(id: recurring.id) if recurring.id

      if recurring.merchant_id.present?
        scope.exists?(merchant_id: recurring.merchant_id)
      else
        scope.exists?(name: recurring.name, merchant_id: nil)
      end
    end
  end
end