How do I get a value from the database?

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Ruby

I am currently developing a web application using Ruby on Rails. I have a table for workers and a table for grades. When a new worker is added to the system the form looks as follows:

<h1>New worker</h1>

<% form_for(@worker) do |f| %>
  <%= f.error_messages %>

  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.label :grade %><br />
    <%= f.text_field :grade %>
  </p>
  <p>
    <%= f.label :remaining_days %><br />
    <%= f.text_field :remaining_days %>
  </p>
  <p>
    <%= f.submit 'Create' %>
  </p>
<% end %>

<%= link_to 'Back', workers_path %>

My grades table contains two columns which are grade and no_of_days.

When a new worker is being added, I need the no_of_days to be added automatically depending on the grade selected.