prevent deleting sections that have registrations or reg_requests in them. Fixes CodeGrade#296, I think

Author: Ben Lerner

app/models/section.rb

@@ -9,6 +9,8 @@ def self.inheritance_column
   delegate :term, to: :course
   has_many :registration_sections
   has_many :registrations, through: :registration_sections
+  has_many :reg_request_sections
+  has_many :reg_requests, through: :reg_request_sections
   has_many :users, through: :registrations
 
   has_many :submission_enabled_toggles
@@ -17,6 +19,28 @@ def self.inheritance_column
   validates :instructor, presence: true
   validates :meeting_time, length: { minimum: 3 }
 
+  before_destroy :ensure_destroyable, prepend: true
+
+  def ensure_destroyable
+    prohibited = nonempty_section?
+    if prohibited
+      errors.add(:base, prohibited)
+      throw :abort
+    end
+  end
+
+  def nonempty_section?
+    reg_count = registration_sections.count
+    reg_req_count = reg_requests.count
+    if reg_count == 0 && reg_req_count == 0
+      return false
+    else
+      reg_msg = "#{'is'.pluralize(reg_count)} #{reg_count} #{'registration'.pluralize(reg_count)}"
+      reg_req_msg = "#{reg_req_count} registration #{'request'.pluralize(reg_req_count)}"
+      return "Cannot delete this section: there #{reg_msg} and #{reg_req_msg}  in it"
+    end
+  end
+
   def to_s(show_instructor: true, show_time: true, show_type: true)
     ans = "#{self.crn}"
     ans = "#{ans} : #{self.instructor.last_name}" if show_instructor

app/views/courses/_section.html.erb

@@ -15,6 +15,10 @@
     <%= f.select :type, Section.types.to_a.map{|t| [t[0].humanize, t[0]]} %>
   </td>
   <td>
+    <% if f.object.nonempty_section? %>
+    <button type="button" disabled title="<%= f.object.nonempty_section? %>" class="btn btn-danger">Remove</button>
+    <% else %>
     <%= link_to_remove_association "Remove", f, class: 'btn btn-danger' %>
+    <% end %>
   </td>
 </tr>

test/integration/request_reg_test.rb

@@ -74,6 +74,38 @@ class RequestRegTest < ActionDispatch::IntegrationTest
     assert new_section.students.include? @john
   end
 
+  test "cannot delete sections with registrations in it" do
+    @lately = create(:user, name: "Johnny Come Lately", first_name: "Johnny", last_name: "Lately")
+    assert_empty @lately.registrations
+    reg_req = @cs101.reg_requests.new(role: Registration::roles[:student], user: @lately, lecture_sections: @section.crn.to_s)
+    reg_req.save!
+    @lately.reload
+    assert_empty @lately.registrations
+    
+    assert_equal [@section], reg_req.sections
+    sign_in @fred
+    get user_path(@lately)
+    assert_response :success
+    @lately.reload
+    assert_empty @lately.registrations
+    assert_not_nil @lately.grouped_registrations
+    assert_equal 0, @lately.grouped_registrations["student"][:count]
+
+    delete accept_course_reg_request_path(@cs101, reg_req)
+    assert_response :redirect
+    follow_redirect!
+    assert_response :success
+    @lately.reload
+    assert_not_empty @lately.registrations
+    assert_not_nil @lately.grouped_registrations
+    assert_equal 1, @lately.grouped_registrations["student"][:count]
+
+    assert_raises(ActiveRecord::RecordNotDestroyed) { @section.destroy! }
+    assert_not @section.destroyed?
+    @lately.reload
+    assert_not_nil @lately.grouped_registrations
+  end
+  
   test "registration via staff page after adding section" do
     sign_in @fred
     new_section = Section.new(course: @cs101,