grabbing values from f.select options_for_select in rails app - ruby-on-rails

Question

Hi in my apps I have a scaffold, were users can select different type of paper and add a paper weight for each type. Then I calculate the running total of paper_weight for each user in the ApplicationController.
I want to be more specific and show the users the average weight of each paper_type they are using. I´m a bit lost since I ´m rather new to rails, can anyone here help me??
thanks in advance
below are my views.
in paper/new.html.erb
<div class="control-group">
<%= f.label :paper_type, class: 'control-label' %><br>
<div class="controls">
<%= f.select(:paper_type, options_for_select([['Skrifstofupappír', 'Skrifstofupappír'], ['Dagblaðapappír', 'Dagblaðapappír'], ['Glans_&_tímaritapappír', 'Glans_&_tímaritapappír'], ['Annað', 'Annað']])) %>
</div>
</div>
<div class="control-group">
<%= f.label :paper_weight, class: 'control-label' %><br>
<div class="controls">
<%= f.number_field :paper_weight %>
</div>
</div>
in ApplicationController.rb
#paper_weight_per_capita = current_user.papers.sum(:paper_weight) / (current_user.profile.staff)

Answer 1

First a tweak.
In your Paper class is where you want to put the array of acceptable values for the paper type. That way you can validate input from perhaps more than just this form in the future. It also allows you to test that code plus putting an array right in the view like that is just not the best practice.
class Paper
PAPER_TYPES = ['Skrifstofupappír', 'Dagblaðapappír', 'Glans_&_tímaritapappír', 'Annað']
...
Then in your view, change to this.
<%= f.select(:paper_type, options_for_select(Paper::PAPER_TYPES)) %>
That doesn't answer your question, just a tweak to make your code cleaner and testable.
Then in your User model
def average_paper_types
self.papers.group(:paper_type).sum(:paper_weight)
end
And in your partial:
<table>
<thead>
<tr>
<% current_user.average_paper_types.each do |pair| %>
<th><%= pair[0] %></th>
<% end %>
</tr>
</thead>
<tbody>
<tr>
<% current_user.average_paper_types.each do |pair| %>
<td><%= pair[1] %></td>
<% end %>
</tr>
</tbody>
</table>

Answer 2

Not positive I understand correctly, but I think what you're trying to do (based on comments) is display the averages of each individual paper type on the index page.
As you're iterating through the different types of paper that belong to the user, you could call a method that performs that calculation. Inside that method you might do something like this:
def paper_weight_by_type(user_id, paper_type)
u = User.find(user_id)
weight_by_type = Paper.where(user_id: user_id, paper_type: paper_type).pluck(:paper_weight)
# then return this
weight_by_type.inject(:+)
end
this is a crude version. This could be an instance method on either user or paper in which case you wouldn't need both arguments. (ie current_user.paper_weight_by_type(paper_type) ...could also make it a class method and just pass both arguments by adding self. in front of def) Also several ways you could probably cut down on the number of queries being run...but this would get the job done. You'd want to call it from the paper loop on the index page however you ended up doing it. I would get it working one way and then improve it from there. Hope this helps.