Doing Subtracting on my Rails App with user input - ruby-on-rails

Question

I'm just starting to learn Rails so please forgive the dumb question. In my web app, I was able to set up working models, forms, and view. A user is able to input their decimal answer and it shows on the web page perfectly. However, I want to subtract one user input from another user input. So if the user inputs 100 in one model "post.price" and 10 in another input model "ratings1.content" > I want it to show 90 in the "fprice.content3". Any help that you guys can give me would be so amazing, I feel like the issue might be in my controller for fprice. I have listed below all my relevant code. Thank you again :)
_form.html "post.price"
<%= simple_form_for #post do |f| %>
<%= f.input :title %>
<%= f.input :image %>
<%= f.input :price %>
<%= f.button :submit %>
<% end %>
_form.html "ratings1s.content"
<%= simple_form_for ([#post, #post.ratings1s.build]) do |f| %>
<%= f.input_field :content %>
<% end %>
_form.html "fprice.content3"
<%= simple_form_for ([#post, #post.fprices.build]) do |f| %>
<%= f.input_field :content3 %>
<% end %>
Rails Controller "fprices"
class FpricesController < ApplicationController
before_action :authenticate_user!
def create
#post = Post.find(params[:post_id])
#fprice = Fprice.create(params[:fprice].permit(:content3))
#fprice.content3 === #post.price - #ratings1.content
#fprice.user_id = current_user.id
#fprice.post_id = #post.id
if #fprice.save
redirect_to post_path(#post)
else
render 'new'
end
end
end
Rails Controller "ratings1s"
class Ratings1sController < ApplicationController
before_action :authenticate_user!
def create
#post = Post.find(params[:post_id])
#ratings1 = Ratings1.create(params[:ratings1].permit(:content))
#ratings1.content *= 10
#ratings1.user_id = current_user.id
#ratings1.post_id = #post.id
if #ratings1.save
redirect_to post_path(#post)
else
render 'new'
end
end
end

Answer 1

It appears that at no point in time are you every defining #ratings1 object, then you call the content method on it while the value of #ratings1 is nil and thats what cause the error. Maybe something like this should help:
class FpricesController < ApplicationController
before_action :authenticate_user!
def create
#post = Post.find(params[:post_id])
#fprice = Fprice.create(params[:fprice].permit(:content3))
#ratings1 = #post.ratings1s.last
#fprice.content3 = #post.price - #ratings1.content
#fprice.user_id = current_user.id
#fprice.post_id = #post.id
if #fprice.save
redirect_to post_path(#post)
else
render 'new'
end
end
end
I think this line is also a the problem
#fprice.content3 === #post.price - #ratings1.content
=== is an equality operator so I don't know why you would have it there replace it with this
#fprice.content3 = #post.price - #ratings1.content