diff --git a/app/controllers/errors_controller.rb b/app/controllers/errors_controller.rb
index 9abd8f6f..e5906078 100644
--- a/app/controllers/errors_controller.rb
+++ b/app/controllers/errors_controller.rb
@@ -2,7 +2,7 @@ class ErrorsController < ApplicationController
   before_action :set_execution_environment
 
   def authorize!
-    authorize(@error || Error.where(execution_environment_id: @execution_environment.id))
+    authorize(@error || @execution_environment.errors)
   end
   private :authorize!
 
diff --git a/app/controllers/exercises_controller.rb b/app/controllers/exercises_controller.rb
index cf951030..8ad4e09d 100644
--- a/app/controllers/exercises_controller.rb
+++ b/app/controllers/exercises_controller.rb
@@ -71,7 +71,7 @@ class ExercisesController < ApplicationController
   private :handle_file_uploads
 
   def implement
-    @submission = Submission.where(exercise_id: @exercise.id, user_id: current_user.id).order('created_at DESC').first
+    @submission = current_user.submissions.where(exercise_id: @exercise.id).order('created_at DESC').first
     @files = (@submission ? @submission.collect_files : @exercise.files).select(&:visible).sort_by(&:name_with_extension)
     @paths = collect_paths(@files)
   end
diff --git a/app/controllers/hints_controller.rb b/app/controllers/hints_controller.rb
index 14145e6f..3f0acf4c 100644
--- a/app/controllers/hints_controller.rb
+++ b/app/controllers/hints_controller.rb
@@ -28,7 +28,7 @@ class HintsController < ApplicationController
   private :hint_params
 
   def index
-    @hints = Hint.where(execution_environment_id: @execution_environment.id).order(:name)
+    @execution_environment.hints.order(:name)
     authorize!
   end
 
diff --git a/app/models/submission.rb b/app/models/submission.rb
index 6cd8b6fa..5a492d34 100644
--- a/app/models/submission.rb
+++ b/app/models/submission.rb
@@ -52,7 +52,7 @@ class Submission < ActiveRecord::Base
   end
 
   def siblings
-    Submission.where(exercise_id: exercise_id, user_id: user_id, user_type: user_type)
+    user.submissions.where(exercise_id: exercise_id)
   end
 
   def to_s
diff --git a/app/policies/exercise_policy.rb b/app/policies/exercise_policy.rb
index ea8fb768..1eb0ab12 100644
--- a/app/policies/exercise_policy.rb
+++ b/app/policies/exercise_policy.rb
@@ -22,7 +22,7 @@ class ExercisePolicy < AdminOrAuthorPolicy
       if @user.admin?
         @scope.all
       elsif @user.internal_user?
-        @scope.where("user_id = #{@user.id} OR public = TRUE OR (team_id IS NOT NULL AND team_id IN (SELECT t.id FROM teams t JOIN internal_users_teams iut ON t.id = iut.team_id WHERE iut.internal_user_id = #{@user.id}))")
+        @scope.where('user_id = ? OR public = TRUE OR (team_id IS NOT NULL AND team_id IN (SELECT t.id FROM teams t JOIN internal_users_teams iut ON t.id = iut.team_id WHERE iut.internal_user_id = ?))', @user.id, @user.id)
       else
         @scope.none
       end
diff --git a/lib/whistleblower.rb b/lib/whistleblower.rb
index 5d5102cf..43876494 100644
--- a/lib/whistleblower.rb
+++ b/lib/whistleblower.rb
@@ -2,7 +2,7 @@ class Whistleblower
   PLACEHOLDER_REGEXP = /\$(\d)/
 
   def find_hint(stderr)
-    Hint.where(execution_environment_id: @execution_environment.id).detect do |hint|
+    @execution_environment.hints.detect do |hint|
       @matches = Regexp.new(hint.regular_expression).match(stderr)
     end
   end