sure/app/controllers/transfers_controller.rb

class TransfersController < ApplicationController
  include StreamExtensions

  before_action :set_transfer, only: %i[show destroy update mark_as_recurring]
  before_action :set_accounts, only: %i[new create]

  def new
    @transfer = Transfer.new
    @from_account_id = params[:from_account_id]
  end

  def show
    @categories = Current.family.categories.alphabetically

    # Whether the current user can hit `mark_as_recurring`: feature flag on,
    # AND they have write access to BOTH transfer endpoints. Gating the
    # view button on this avoids showing a CTA that the controller would
    # reject via `require_account_permission!` for read-only sharers.
    endpoint_ids = [ @transfer.from_account&.id, @transfer.to_account&.id ].compact
    writable_endpoint_count = Account.writable_by(Current.user).where(id: endpoint_ids).distinct.count
    @can_mark_as_recurring_transfer =
      !Current.family.recurring_transactions_disabled? &&
      endpoint_ids.size == 2 &&
      writable_endpoint_count == 2
  end

  def create
    # Validate user has write access to both accounts
    source_account = accessible_accounts.find(transfer_params[:from_account_id])
    destination_account = accessible_accounts.find(transfer_params[:to_account_id])

    return unless require_account_permission!(source_account, redirect_path: transactions_path)
    return unless require_account_permission!(destination_account, redirect_path: transactions_path)

    @transfer = Transfer::Creator.new(
      family: Current.family,
      source_account_id: source_account.id,
      destination_account_id: destination_account.id,
      date: transfer_params[:date].present? ? Date.parse(transfer_params[:date]) : Date.current,
      amount: transfer_params[:amount].to_d,
      exchange_rate: transfer_params[:exchange_rate].presence&.to_d,
      source_fee_amount: transfer_params[:source_fee_amount],
      destination_fee_amount: transfer_params[:destination_fee_amount]
    ).create

    if @transfer.persisted?
      success_message = "Transfer created"
      respond_to do |format|
        format.html { redirect_back_or_to transactions_path, notice: success_message }
        format.turbo_stream { stream_redirect_back_or_to transactions_path, notice: success_message }
      end
    else
      @from_account_id = transfer_params[:from_account_id]
      render :new, status: :unprocessable_entity
    end
  rescue Money::ConversionError
    @transfer ||= Transfer.new
    @transfer.errors.add(:base, "Exchange rate unavailable for selected currencies and date")
    set_accounts
    render :new, status: :unprocessable_entity
  rescue ArgumentError
    @transfer ||= Transfer.new
    @transfer.errors.add(:date, "is invalid")
    set_accounts
    render :new, status: :unprocessable_entity
  end

  def update
    outflow_account = @transfer.outflow_transaction.entry.account
    return unless require_account_permission!(outflow_account, redirect_path: transactions_url)

    Transfer.transaction do
      update_transfer_status
      update_transfer_fees_and_amount
      update_transfer_details unless transfer_update_params[:status] == "rejected"
    end

    respond_to do |format|
      format.html { redirect_back_or_to transactions_url, notice: t(".success") }
      format.turbo_stream
    end
  end

  def destroy
    outflow_account = @transfer.outflow_transaction.entry.account
    return unless require_account_permission!(outflow_account, redirect_path: transactions_url)

    @transfer.destroy!
    redirect_back_or_to transactions_url, notice: t(".success")
  end

  def mark_as_recurring
    if Current.family.recurring_transactions_disabled?
      flash[:alert] = t("recurring_transactions.transfer_feature_disabled")
      redirect_back_or_to transactions_path
      return
    end

    source_account      = @transfer.from_account
    destination_account = @transfer.to_account

    if source_account.nil? || destination_account.nil?
      flash[:alert] = t("recurring_transactions.unexpected_error")
      redirect_back_or_to transactions_path
      return
    end

    return unless require_account_permission!(source_account)
    return unless require_account_permission!(destination_account)

    existing = Current.family.recurring_transactions.find_by(
      account_id: source_account.id,
      destination_account_id: destination_account.id,
      amount: @transfer.outflow_transaction.entry.amount,
      currency: @transfer.outflow_transaction.entry.currency
    )

    if existing
      flash[:alert] = t("recurring_transactions.transfer_already_exists")
      respond_to do |format|
        format.html { redirect_back_or_to transactions_path }
      end
      return
    end

    begin
      RecurringTransaction.create_from_transfer(@transfer)
      flash[:notice] = t("recurring_transactions.transfer_marked_as_recurring")
      respond_to do |format|
        format.html { redirect_back_or_to transactions_path }
      end
    rescue ActiveRecord::RecordInvalid, ActiveRecord::RecordNotUnique
      # RecordNotUnique covers the race window between `find_by` and `create!`
      # (the partial unique index protects us at the DB level).
      flash[:alert] = t("recurring_transactions.transfer_creation_failed")
      respond_to do |format|
        format.html { redirect_back_or_to transactions_path }
      end
    rescue StandardError => e
      Rails.logger.error(
        "transfers#mark_as_recurring failed: #{e.class} #{e.message} " \
        "(transfer=#{@transfer&.id} family=#{Current.family&.id} user=#{Current.user&.id})"
      )
      flash[:alert] = t("recurring_transactions.unexpected_error")
      respond_to do |format|
        format.html { redirect_back_or_to transactions_path }
      end
    end
  end

  private
    def set_transfer
      # Finds the transfer and ensures the user has access to it
      accessible_transaction_ids = Current.family.transactions
        .joins(entry: :account)
        .merge(Account.accessible_by(Current.user))
        .select(:id)

      @transfer = Transfer
                    .where(id: params[:id])
                    .where(inflow_transaction_id: accessible_transaction_ids)
                    .first!
    end

    def transfer_params
      params.require(:transfer).permit(:from_account_id, :to_account_id, :amount, :date, :name, :excluded, :exchange_rate, :source_fee_amount, :destination_fee_amount)
    end

    def set_accounts
      @accounts = accessible_accounts
        .alphabetically
        .includes(
          :account_providers,
          logo_attachment: :blob
        )
    end

    def transfer_update_params
      params.require(:transfer).permit(:notes, :status, :category_id, :amount, :source_fee_amount, :destination_fee_amount)
    end

    def update_transfer_status
      if transfer_update_params[:status] == "rejected"
        @transfer.reject!
      elsif transfer_update_params[:status] == "confirmed"
        @transfer.confirm!
      end
    end

    def update_transfer_details
      @transfer.outflow_transaction.update!(category_id: transfer_update_params[:category_id])
      @transfer.update!(notes: transfer_update_params[:notes])
    end

    def update_transfer_fees_and_amount
      new_amount = transfer_update_params[:amount]
      new_source_fee = transfer_update_params[:source_fee_amount]
      new_destination_fee = transfer_update_params[:destination_fee_amount]

      amount_changed = new_amount.present? && new_amount.to_d != @transfer.amount.to_d
      source_fee_changed = new_source_fee.present? && new_source_fee.to_d != @transfer.source_fee_amount.to_d
      dest_fee_changed = new_destination_fee.present? && new_destination_fee.to_d != @transfer.destination_fee_amount.to_d

      return unless amount_changed || source_fee_changed || dest_fee_changed

      @transfer.amount = new_amount.to_d if amount_changed
      @transfer.source_fee_amount = new_source_fee.to_d if source_fee_changed
      @transfer.destination_fee_amount = new_destination_fee.to_d if dest_fee_changed

      # Recompute outflow entry (always principal only)
      if amount_changed
        outflow_entry = @transfer.outflow_transaction.entry
        outflow_entry.amount = @transfer.amount
        outflow_entry.save!
      end

      # Recompute inflow entry (always principal converted, no fee baked in)
      if amount_changed
        inflow_entry = @transfer.inflow_transaction.entry
        converted = Money.new(@transfer.amount, @transfer.from_account.currency)
                      .exchange_to(@transfer.to_account.currency, date: @transfer.date)
        inflow_entry.amount = -(converted.amount)
        inflow_entry.save!
      end

      # Update source fee transaction
      if source_fee_changed
        update_fee_transaction(
          account: @transfer.from_account,
          old_fee: @transfer.source_fee_amount_before_last_save || @transfer.source_fee_amount,
          new_fee: @transfer.source_fee_amount,
          name: "Transfer fee — #{@transfer.name}"
        )
      end

      # Update destination fee transaction
      if dest_fee_changed
        update_fee_transaction(
          account: @transfer.to_account,
          old_fee: @transfer.destination_fee_amount_before_last_save || @transfer.destination_fee_amount,
          new_fee: @transfer.destination_fee_amount,
          name: "Transfer fee — #{@transfer.name}"
        )
      end

      @transfer.save!
    end

    def update_fee_transaction(account:, old_fee:, new_fee:, name:)
      if old_fee > 0 && new_fee > 0
        fee_tx = @transfer.fee_transactions.find { |t| t.entry.account_id == account.id }
        if fee_tx
          fee_tx.entry.update!(amount: new_fee)
        end
      elsif old_fee > 0 && new_fee == 0
        fee_tx = @transfer.fee_transactions.find { |t| t.entry.account_id == account.id }
        fee_tx&.destroy!
      elsif old_fee == 0 && new_fee > 0
        fee_category = account.family.categories.find_or_create_by!(name: I18n.t("models.category.defaults.fees"))
        fee_tx = Transaction.new(
          kind: "standard",
          category: fee_category,
          entry: account.entries.build(
            amount: new_fee,
            currency: account.currency,
            date: @transfer.date,
            name: name,
          )
        )
        fee_tx.save!
        @transfer.fee_transactions << fee_tx
      end
    end
end