Generalize breadcrumbs for nested resources

* Make programming groups breadcrumb navigation clickable

app/models/programming_group.rb

@@ -28,8 +28,8 @@ class ProgrammingGroup < ApplicationRecord
     false
   end
 
-  def self.nested_resource?
-    true
+  def self.parent_resource
+    Exercise
   end
 
   def programming_group?

app/views/application/_breadcrumbs_and_title.html.slim

@@ -1,10 +1,19 @@
 - model = controller_path.classify.constantize rescue nil
 - if model
   - object = model.find_by(id: params[:id])
-  - if model.try(:nested_resource?)
-    - root_element = model.model_name.human(count: 2)
-    - if object
-      - current_element = object
+  - if (parent_model = model.try(:parent_resource))
+    - parent_route_key = parent_model.model_name.singular_route_key
+    - if params["#{parent_route_key}_id"].present?
+      - parent_object = object.try(parent_route_key) || parent_model.find_by(id: params["#{parent_route_key}_id"])
+      - parent_element = link_to_if(current_user && policy(parent_object).show?, parent_object, send(:"#{parent_route_key}_path", parent_object))
+      - parent_root_element = link_to_if(current_user && policy(parent_model).index?, parent_model.model_name.human(count: 2), send(:"#{parent_model.model_name.collection}_path"))
+      - root_element = link_to_if(current_user && policy(model).index?, model.model_name.human(count: 2), send(:"#{parent_route_key}_#{model.model_name.collection}_path", parent_object))
+      - if object
+        - current_element = link_to_if(current_user && policy(object).show?, object, send(:"#{parent_route_key}_#{model.model_name.singular}_path", parent_object, object))
+    - else
+      - root_element = link_to_if(current_user && policy(model).index?, model.model_name.human(count: 2), send(:"#{model.model_name.collection}_path"))
+      - if object
+        - current_element = link_to_if(current_user && policy(object).show?, object, send(:"#{model.model_name.singular}_path", object))
   - else
     - root_element = link_to_if(current_user && policy(model).index?, model.model_name.human(count: 2), send(:"#{model.model_name.collection}_path"))
     - if object
@@ -21,6 +30,12 @@
 - content_for :breadcrumbs do
   .container.mb-4
     ul.breadcrumb.bg-body-secondary.px-3.py-2
+      - if defined?(parent_root_element) && parent_root_element.present?
+        li.breadcrumb-item.small
+          = parent_root_element
+        li.breadcrumb-item.small
+          = parent_element
+        - title = "#{parent_object} - #{title}"
       - if root_element.present?
         li.breadcrumb-item.small
           = root_element

db/migrate/20181129093207_drop_errors.rb

@@ -11,8 +11,8 @@ class DropErrors < ActiveRecord::Migration[5.2]
 
       validates :message, presence: true
 
-      def self.nested_resource?
-        true
+      def self.parent_resource
+        ExecutionEnvironment
       end
 
       delegate :to_s, to: :id