Running into undefined method from model class in view - ruby-on-rails

Question

I have a method called calculation_of_total_cost in model Tippy
It's running into problems being called in index.html.erb via tippies views directory.
This is the error I receive: undefined method*' for nil:NilClass`
I have googled it, and now understand that it is the result of the one of the variables being nil.
How do I resolve this, i.e, how do I make the method work in index.html.erb? This is index view that I am calling it from, so I need an instance method, not class, right?
Also, addendum: this same method works fine in show.html.erb
show.html.erb
<br/><br/>
<h1 class="text-center">Your Total Cost</h1>
<br/><br />
<table class="table table-striped">
<tr>
<td>
Cost of Your Meal:
</td>
<td>
<%= humanized_money_with_symbol #tippy.cost %>
</td>
</tr>
<tr>
<td>
Tip You Picked:
</td>
<td>
<%= number_to_percentage(#tippy.tip * 100, format: "%n%", precision: 0) %>
</td>
</tr>
<tr>
<td>
The Total Cost:
</td>
<td>
<%= humanized_money_with_symbol #tippy.calculation_of_total_cost %>
</td>
</tr>
</table>
<%= link_to 'New Tippy', new_tippy_path %>
<%= link_to "Index", tippies_path %>
Here is the Tippy model:
class Tippy < ApplicationRecord
validates :tip, presence: true
validates :cost, presence: true
#monetize :tip_cents
monetize :cost_cents, :numericality => {:greater_than => 0}
TIP_CHOICES = { "10%" => ".10", "20%" => ".20", "30%" => ".30", "40%" => ".40", "50%" => ".50",
"60%" => ".60", "70%" => ".70", "80%" => ".80", "90%" => ".90" }
def calculation_of_total_cost
cost + (tip * cost)
end
end
Here is the index.html.erb file
<p id="notice"><%= notice %></p>
<h1>Tippies</h1>
<table>
<thead>
<tr>
<th>Tip</th>
<th>Cost</th>
<th>Total</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% #tippies.each do |tippy| %>
<tr>
<td><%= tippy.tip %></td>
<td><%= tippy.cost %></td>
<td><%= tippy.calculation_of_total_cost %></td>
<td><%= link_to 'Show', tippy %></td>
<td><%= link_to 'Edit', edit_tippy_path(tippy) %></td>
<td><%= link_to 'Destroy', tippy, method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
<br>
<%= link_to 'New Tippy', new_tippy_path %>
Tippy Controller
class TippiesController < ApplicationController
#before_action :set_tippy, only: [:show, :edit, :update, :destroy]
# GET /tippies
# GET /tippies.json
def index
#tippies = Tippy.all
end
# GET /tippies/1
# GET /tippies/1.json
def show
##calculation_of_total_cost
end
# GET /tippies/new
def new
#tippy = Tippy.new
end
# GET /tippies/1/edit
def edit
end
# POST /tippies
# POST /tippies.json
def create
#tippy = Tippy.new(tippy_params)
respond_to do |format|
if #tippy.save
format.html { redirect_to #tippy, notice: 'Tippy was successfully created.' }
format.json { render :show, status: :created, location: #tippy }
else
format.html { render :new }
format.json { render json: #tippy.errors, status: :unprocessable_entity }
end
end
end
# PATCH/PUT /tippies/1
# PATCH/PUT /tippies/1.json
def update
respond_to do |format|
if #tippy.update(tippy_params)
format.html { redirect_to #tippy, notice: 'Tippy was successfully updated.' }
format.json { render :show, status: :ok, location: #tippy }
else
format.html { render :edit }
format.json { render json: #tippy.errors, status: :unprocessable_entity }
end
end
end
# DELETE /tippies/1
# DELETE /tippies/1.json
def destroy
#tippy.destroy
respond_to do |format|
format.html { redirect_to tippies_url, notice: 'Tippy was successfully destroyed.' }
format.json { head :no_content }
end
end
private
# Use callbacks to share common setup or constraints between actions.
def set_tippy
#tippy = Tippy.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def tippy_params
params.require(:tippy).permit(:tip, :cost)
end
end

Answer 1

To solve this problem you need to set a binding.pry or a breakpoint at this line of index.html.erb, so that we can understand in the loop you are executing why tippy is getting value of nil.
You need to install pry gem.
Please also share the values of #tippies and the details of the other variable in the loop that fails, because tippy=nil.
An alternative for pry is just printing the value of the variable in the log with puts tippy.calculation_of_total_cost.
Right now I am guess is that #tippies which includes all #tippy in your tippies table, could have one field that has calculation of total cost = nil. To verifiy this you should check with the debug the value of tippy and of tippy.calculation_of_total_cost in the index.html.erb view.
<% #tippies.each do |tippy| %>
<tr>
<% binding.pry %>
<td><%= tippy.tip %></td>
<td><%= tippy.cost %></td>
<td><%= tippy.calculation_of_total_cost %></td>
<td><%= link_to 'Show', tippy %></td>
<td><%= link_to 'Edit', edit_tippy_path(tippy) %></td>
<td><%= link_to 'Destroy', tippy, method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
also it is a good idea to inspect show.html.erb as there it is working.
def calculation_of_total_cost
cost + (tip * cost)
end