account module

module AccountBalances
  def credit_to_balance(account, credit)
    if account.update_attributes(balance: account.balance + credit.amount)
      account
    else
      false
    end
  end
  
  def debit_from_balance(account, debit)
    if account.balance < debit.amount
      #flash[:notice] = "debit.amount > account.balance"
      false
    else
      account.update_attributes(balance: account.balance -= debit.amount)
      account
    end
  end
end

debits controller

class DebitsController < ApplicationController
  def new
    @current_account = signed_in_user.accounts.find(params[:account_id])
    @new_debit = @current_account.debits.build(params[:debit])
  end
  def create
    @current_account = signed_in_user.accounts.find(params[:account_id])
    @new_debit = @current_account.debits.build(params[:debit])
    @new_debit = @current_account.debits.debit_from_balance(@current_account, @new_debit)
    if @new_debit.save
      redirect_to account_path(@current_account)
    else 
      render "new"
    end
  end
end