school-platform/app/views/assignments/_form.html.erb

<%= bootstrap_form_for(@assignment) do |f| %>
  <%= render 'shared/error_messages', object: f.object %>

  <% unless action_name == 'create' %>
    <%= f.select :course_id, options_for_select(current_user.courses.merge(CourseParticipation.teachers).map{ |course| [course.name, course.id] }) %>
  <% else %>
    <%= f.text_field :course, value: @assignment.course.name, disabled: true %>
  <% end %>

  <%= f.text_field :name %>

  <%= f.datetime_field :due_at %>

  <%= f.text_area :description %>

  <%= f.submit yield(:button_text), class: "btn btn-primary" %>
<% end %>