diff --git a/db/migrate/20140826073318_sorcery_core.rb b/db/migrate/20140826073318_sorcery_core.rb
index ce978f4f..8fc602dd 100644
--- a/db/migrate/20140826073318_sorcery_core.rb
+++ b/db/migrate/20140826073318_sorcery_core.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class SorceryCore < ActiveRecord::Migration[4.2]
+  class InternalUser < ApplicationRecord
+  end
+
   def change
     InternalUser.delete_all
     add_column :internal_users, :crypted_password, :string, null: false
diff --git a/db/migrate/20140918063522_add_hashed_content_to_files.rb b/db/migrate/20140918063522_add_hashed_content_to_files.rb
index 8b5393d4..b338b3bc 100644
--- a/db/migrate/20140918063522_add_hashed_content_to_files.rb
+++ b/db/migrate/20140918063522_add_hashed_content_to_files.rb
@@ -1,6 +1,20 @@
 # frozen_string_literal: true
 
 class AddHashedContentToFiles < ActiveRecord::Migration[4.2]
+  class CodeOcean::File < ApplicationRecord
+    before_validation :hash_content, if: :content_present?
+
+    private
+
+    def content_present?
+      content? || native_file?
+    end
+
+    def hash_content
+      self.hashed_content = Digest::MD5.new.hexdigest(read || '')
+    end
+  end
+
   def change
     add_column :files, :hashed_content, :string
 
diff --git a/db/migrate/20150204080832_add_pool_size_to_execution_environments.rb b/db/migrate/20150204080832_add_pool_size_to_execution_environments.rb
index 11b7dc79..0df5f661 100644
--- a/db/migrate/20150204080832_add_pool_size_to_execution_environments.rb
+++ b/db/migrate/20150204080832_add_pool_size_to_execution_environments.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class AddPoolSizeToExecutionEnvironments < ActiveRecord::Migration[4.2]
+  class ExecutionEnvironment < ApplicationRecord
+  end
+
   def change
     add_column :execution_environments, :pool_size, :integer
 
diff --git a/db/migrate/20150317083739_add_memory_limit_to_execution_environments.rb b/db/migrate/20150317083739_add_memory_limit_to_execution_environments.rb
index 05d4a7b7..9d67506a 100644
--- a/db/migrate/20150317083739_add_memory_limit_to_execution_environments.rb
+++ b/db/migrate/20150317083739_add_memory_limit_to_execution_environments.rb
@@ -1,6 +1,10 @@
 # frozen_string_literal: true
 
 class AddMemoryLimitToExecutionEnvironments < ActiveRecord::Migration[4.2]
+  class ExecutionEnvironment < ApplicationRecord
+    DEFAULT_MEMORY_LIMIT = 256
+  end
+
   def change
     add_column :execution_environments, :memory_limit, :integer
 
diff --git a/db/migrate/20150317115338_add_network_enabled_to_execution_environments.rb b/db/migrate/20150317115338_add_network_enabled_to_execution_environments.rb
index 182cc66e..30406871 100644
--- a/db/migrate/20150317115338_add_network_enabled_to_execution_environments.rb
+++ b/db/migrate/20150317115338_add_network_enabled_to_execution_environments.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class AddNetworkEnabledToExecutionEnvironments < ActiveRecord::Migration[4.2]
+  class ExecutionEnvironment < ApplicationRecord
+  end
+
   def change
     add_column :execution_environments, :network_enabled, :boolean
 
diff --git a/db/migrate/20160630154310_add_submission_to_request_for_comments.rb b/db/migrate/20160630154310_add_submission_to_request_for_comments.rb
index 65dac9f1..3396f4c2 100644
--- a/db/migrate/20160630154310_add_submission_to_request_for_comments.rb
+++ b/db/migrate/20160630154310_add_submission_to_request_for_comments.rb
@@ -3,31 +3,32 @@
 class AddSubmissionToRequestForComments < ActiveRecord::Migration[4.2]
   def change
     add_reference :request_for_comments, :submission
+
+    up_only do
+      execute <<-SQL.squish
+        UPDATE request_for_comments
+        SET submission_id = sub.submission_id_external
+        FROM
+        (SELECT s.id AS submission_id_external,
+                        rfc.id AS rfc_id,
+                                  s.created_at AS submission_created_at,
+                                                  rfc.created_at AS rfc_crea
+        FROM submissions s,
+                         request_for_comments rfc
+        WHERE s.user_id = rfc.user_id
+        AND s.exercise_id = rfc.exercise_id
+        AND rfc.created_at + interval '2 hours' > s.created_at
+        AND s.created_at =
+                (SELECT MAX(created_at)
+                FROM submissions
+                WHERE exercise_id = s.exercise_id
+                AND user_id = s.user_id
+                AND rfc.created_at + interval '2 hours' > created_at
+                GROUP BY s.exercise_id,
+                         s.user_id)) as sub
+        WHERE id = sub.rfc_id
+        AND submission_id IS NULL;
+      SQL
+    end
   end
 end
-
-# We issued the following on the database to add the submission_ids for existing entries
-#
-# UPDATE request_for_comments
-# SET submission_id = sub.submission_id_external
-# FROM
-# (SELECT s.id AS submission_id_external,
-#                 rfc.id AS rfc_id,
-#                           s.created_at AS submission_created_at,
-#                                           rfc.created_at AS rfc_created_at
-# FROM submissions s,
-#                  request_for_comments rfc
-# WHERE s.user_id = rfc.user_id
-# AND s.exercise_id = rfc.exercise_id
-# AND rfc.created_at + interval '2 hours' > s.created_at
-# AND s.created_at =
-#         (SELECT MAX(created_at)
-#         FROM submissions
-#         WHERE exercise_id = s.exercise_id
-#         AND user_id = s.user_id
-#         AND rfc.created_at + interval '2 hours' > created_at
-#         GROUP BY s.exercise_id,
-#                  s.user_id)) as sub
-# WHERE id = sub.rfc_id
-# AND submission_id IS NULL;
-#
diff --git a/db/migrate/20170403162848_set_default_for_request_for_comment_solved.rb b/db/migrate/20170403162848_set_default_for_request_for_comment_solved.rb
index bc120c46..81583f44 100644
--- a/db/migrate/20170403162848_set_default_for_request_for_comment_solved.rb
+++ b/db/migrate/20170403162848_set_default_for_request_for_comment_solved.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class SetDefaultForRequestForCommentSolved < ActiveRecord::Migration[4.2]
+  class RequestForComment < ApplicationRecord
+  end
+
   def change
     change_column_default :request_for_comments, :solved, false
     RequestForComment.where(solved: nil).update(solved: false)
diff --git a/db/migrate/20170830083601_add_cause_to_testruns.rb b/db/migrate/20170830083601_add_cause_to_testruns.rb
index 73de7e9c..291b572d 100644
--- a/db/migrate/20170830083601_add_cause_to_testruns.rb
+++ b/db/migrate/20170830083601_add_cause_to_testruns.rb
@@ -1,6 +1,13 @@
 # frozen_string_literal: true
 
 class AddCauseToTestruns < ActiveRecord::Migration[4.2]
+  class Testrun < ApplicationRecord
+    belongs_to :submission, optional: true
+  end
+
+  class Submission < ApplicationRecord
+  end
+
   def up
     add_column :testruns, :cause, :string
     Testrun.reset_column_information
diff --git a/db/migrate/20180130172021_add_reached_full_score_to_request_for_comment.rb b/db/migrate/20180130172021_add_reached_full_score_to_request_for_comment.rb
index 79ecc350..decda30d 100644
--- a/db/migrate/20180130172021_add_reached_full_score_to_request_for_comment.rb
+++ b/db/migrate/20180130172021_add_reached_full_score_to_request_for_comment.rb
@@ -1,6 +1,57 @@
 # frozen_string_literal: true
 
 class AddReachedFullScoreToRequestForComment < ActiveRecord::Migration[4.2]
+  class RequestForComment < ApplicationRecord
+    belongs_to :submission, optional: true
+    belongs_to :user, optional: true
+  end
+
+  class Submission < ApplicationRecord
+    belongs_to :exercise
+    belongs_to :user, polymorphic: true
+  end
+
+  class Exercise < ApplicationRecord
+    has_many :files, as: :context, class_name: 'CodeOcean::File'
+    has_many :submissions
+
+    def solved_by?(user)
+      maximum_score(user).to_i == maximum_score.to_i
+    end
+
+    def maximum_score(user)
+      if user
+        submissions
+          .where(user:, cause: %w[submit assess])
+          .where.not(score: nil)
+          .order(score: :desc)
+          .first&.score || 0
+      else
+        @maximum_score ||= if files.loaded?
+                             files.filter(&:teacher_defined_assessment?).pluck(:weight).sum
+                           else
+                             files.teacher_defined_assessments.sum(:weight)
+                           end
+      end
+    end
+  end
+
+  class CodeOcean::File < ApplicationRecord
+    belongs_to :context, polymorphic: true
+    scope :teacher_defined_assessments, -> { where(role: %w[teacher_defined_test teacher_defined_linter]) }
+
+    ROLES = %w[regular_file main_file reference_implementation executable_file teacher_defined_test user_defined_file
+               user_defined_test teacher_defined_linter].freeze
+
+    def teacher_defined_assessment?
+      teacher_defined_test? || teacher_defined_linter?
+    end
+
+    ROLES.each do |role|
+      define_method(:"#{role}?") { self.role == role }
+    end
+  end
+
   def up
     add_column :request_for_comments, :full_score_reached, :boolean, default: false
     RequestForComment.find_each do |rfc|
diff --git a/db/migrate/20181119161514_add_user_to_proxy_exercise.rb b/db/migrate/20181119161514_add_user_to_proxy_exercise.rb
index 59380ea3..3fa3f5f3 100644
--- a/db/migrate/20181119161514_add_user_to_proxy_exercise.rb
+++ b/db/migrate/20181119161514_add_user_to_proxy_exercise.rb
@@ -1,6 +1,13 @@
 # frozen_string_literal: true
 
 class AddUserToProxyExercise < ActiveRecord::Migration[5.2]
+  class Internaluser < ApplicationRecord
+  end
+
+  class ProxyExercise < ApplicationRecord
+    belongs_to :user, polymorphic: true
+  end
+
   def change
     add_reference :proxy_exercises, :user, polymorphic: true, index: true
     add_column :proxy_exercises, :public, :boolean, null: false, default: false
diff --git a/db/migrate/20181126163428_add_user_type_to_remote_evaluation_mappings.rb b/db/migrate/20181126163428_add_user_type_to_remote_evaluation_mappings.rb
index a3ab1a69..785cca3e 100644
--- a/db/migrate/20181126163428_add_user_type_to_remote_evaluation_mappings.rb
+++ b/db/migrate/20181126163428_add_user_type_to_remote_evaluation_mappings.rb
@@ -1,6 +1,10 @@
 # frozen_string_literal: true
 
 class AddUserTypeToRemoteEvaluationMappings < ActiveRecord::Migration[5.2]
+  class RemoteEvaluationMapping < ApplicationRecord
+    belongs_to :user, polymorphic: true
+  end
+
   def change
     add_column :remote_evaluation_mappings, :user_type, :string
     # Update all existing records and set user_type to `ExternalUser` (safe way to prevent any function loss).
diff --git a/db/migrate/20201019090123_add_normalized_score_and_submission_to_user_exercise_feedback.rb b/db/migrate/20201019090123_add_normalized_score_and_submission_to_user_exercise_feedback.rb
index 04a55e3c..061966a0 100644
--- a/db/migrate/20201019090123_add_normalized_score_and_submission_to_user_exercise_feedback.rb
+++ b/db/migrate/20201019090123_add_normalized_score_and_submission_to_user_exercise_feedback.rb
@@ -1,6 +1,18 @@
 # frozen_string_literal: true
 
 class AddNormalizedScoreAndSubmissionToUserExerciseFeedback < ActiveRecord::Migration[5.2]
+  class UserExerciseFeedback < ApplicationRecord
+    belongs_to :submission
+    belongs_to :exercise
+  end
+
+  class Submission < ApplicationRecord
+    belongs_to :user, polymorphic: true
+  end
+
+  class Exercise < ApplicationRecord
+  end
+
   def change
     add_column :user_exercise_feedbacks, :normalized_score, :float
     add_reference :user_exercise_feedbacks, :submission, foreign_key: true
@@ -9,7 +21,7 @@ class AddNormalizedScoreAndSubmissionToUserExerciseFeedback < ActiveRecord::Migr
     ActiveRecord::Base.record_timestamps = false
     UserExerciseFeedback.find_each do |uef|
       latest_submission = Submission
-        .where(user_id: uef.user_id, user_type: uef.user_type, exercise_id: uef.exercise_id)
+        .where(user: uef.user, exercise: uef.exercise)
         .where(created_at: ...uef.updated_at)
         .order(created_at: :desc).first
 
diff --git a/db/migrate/20210602071834_change_type_of_exposed_ports_in_execution_environment.rb b/db/migrate/20210602071834_change_type_of_exposed_ports_in_execution_environment.rb
index 6715c59a..d88ab604 100644
--- a/db/migrate/20210602071834_change_type_of_exposed_ports_in_execution_environment.rb
+++ b/db/migrate/20210602071834_change_type_of_exposed_ports_in_execution_environment.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class ChangeTypeOfExposedPortsInExecutionEnvironment < ActiveRecord::Migration[6.1]
+  class ExecutionEnvironment < ApplicationRecord
+  end
+
   # rubocop:disable Rails/SkipsModelValidations:
   def up
     rename_column :execution_environments, :exposed_ports, :exposed_ports_migration
diff --git a/db/migrate/20211114145024_create_tips_intervention.rb b/db/migrate/20211114145024_create_tips_intervention.rb
index b5938e34..d7d22d41 100644
--- a/db/migrate/20211114145024_create_tips_intervention.rb
+++ b/db/migrate/20211114145024_create_tips_intervention.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class CreateTipsIntervention < ActiveRecord::Migration[6.1]
+  class Intervention < ApplicationRecord
+  end
+
   def change
     Intervention.find_or_create_by(name: 'TipsIntervention')
   end
diff --git a/db/migrate/20220415215112_migrate_testruns.rb b/db/migrate/20220415215112_migrate_testruns.rb
index 050d628b..218f5526 100644
--- a/db/migrate/20220415215112_migrate_testruns.rb
+++ b/db/migrate/20220415215112_migrate_testruns.rb
@@ -1,6 +1,19 @@
 # frozen_string_literal: true
 
 class MigrateTestruns < ActiveRecord::Migration[6.1]
+  class Testrun < ApplicationRecord
+    belongs_to :submission, optional: true
+    has_many :testrun_messages
+  end
+
+  class Submission < ApplicationRecord
+    has_many :testruns
+  end
+
+  class TestrunMessage < ApplicationRecord
+    belongs_to :testrun
+  end
+
   # We are not changing any tables but only backfilling data.
   disable_ddl_transaction!
 
diff --git a/db/migrate/20220906142550_migrate_permissions_to_study_group.rb b/db/migrate/20220906142550_migrate_permissions_to_study_group.rb
index f35d13df..aed1846a 100644
--- a/db/migrate/20220906142550_migrate_permissions_to_study_group.rb
+++ b/db/migrate/20220906142550_migrate_permissions_to_study_group.rb
@@ -1,6 +1,32 @@
 # frozen_string_literal: true
 
 class MigratePermissionsToStudyGroup < ActiveRecord::Migration[6.1]
+  class Consumer < ApplicationRecord
+    has_many :study_groups
+    has_many :internal_users
+    has_many :external_users
+  end
+
+  class StudyGroup < ApplicationRecord
+    belongs_to :consumer
+    has_many :study_group_memberships
+  end
+
+  class InternalUser < ApplicationRecord
+    belongs_to :consumer
+    has_many :study_group_memberships, as: :user
+  end
+
+  class ExternalUser < ApplicationRecord
+    belongs_to :consumer
+    has_many :study_group_memberships, as: :user
+  end
+
+  class StudyGroupMembership < ApplicationRecord
+    belongs_to :study_group
+    belongs_to :user, polymorphic: true
+  end
+
   # rubocop:disable Rails/SkipsModelValidations
   def up
     create_default_groups
diff --git a/db/migrate/20230206203117_migrate_filetype_extensions_not_nil.rb b/db/migrate/20230206203117_migrate_filetype_extensions_not_nil.rb
index 485d5542..b326de4f 100644
--- a/db/migrate/20230206203117_migrate_filetype_extensions_not_nil.rb
+++ b/db/migrate/20230206203117_migrate_filetype_extensions_not_nil.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class MigrateFiletypeExtensionsNotNil < ActiveRecord::Migration[7.0]
+  class FileType < ApplicationRecord
+  end
+
   def change
     FileType.all.find_all {|file_type| file_type.file_extension.nil? }.each do |file_type|
       file_type.update file_extension: ''
diff --git a/db/migrate/20230819084917_unify_lti_parameters.rb b/db/migrate/20230819084917_unify_lti_parameters.rb
index ce8ec6b1..5def508b 100644
--- a/db/migrate/20230819084917_unify_lti_parameters.rb
+++ b/db/migrate/20230819084917_unify_lti_parameters.rb
@@ -1,6 +1,19 @@
 # frozen_string_literal: true
 
 class UnifyLtiParameters < ActiveRecord::Migration[7.0]
+  class LtiParameter < ApplicationRecord
+    belongs_to :external_user
+    belongs_to :exercise
+  end
+
+  class ExternalUser < ApplicationRecord
+    has_many :lti_parameters
+  end
+
+  class Exercise < ApplicationRecord
+    has_many :lti_parameters
+  end
+
   def change
     reversible do |dir|
       dir.up do
@@ -35,10 +48,4 @@ class UnifyLtiParameters < ActiveRecord::Migration[7.0]
 
     add_index :lti_parameters, %i[external_user_id study_group_id exercise_id], unique: true, name: 'index_lti_params_on_external_user_and_study_group_and_exercise'
   end
-
-  class LtiParameter < ActiveRecord::Base; end
-
-  class ExternalUser < ActiveRecord::Base; end
-
-  class Exercise < ActiveRecord::Base; end
 end
diff --git a/db/migrate/20230821062901_add_foreign_keys_to_anomaly_notifications.rb b/db/migrate/20230821062901_add_foreign_keys_to_anomaly_notifications.rb
index b81f8824..eb8eef8c 100644
--- a/db/migrate/20230821062901_add_foreign_keys_to_anomaly_notifications.rb
+++ b/db/migrate/20230821062901_add_foreign_keys_to_anomaly_notifications.rb
@@ -1,6 +1,14 @@
 # frozen_string_literal: true
 
 class AddForeignKeysToAnomalyNotifications < ActiveRecord::Migration[7.0]
+  class AnomalyNotification < ApplicationRecord
+    belongs_to :exercise
+  end
+
+  class Exercise < ApplicationRecord
+    has_many :anomaly_notifications
+  end
+
   def change
     up_only do
       # We cannot add a foreign key to a table that has rows that violate the constraint.
@@ -16,8 +24,4 @@ class AddForeignKeysToAnomalyNotifications < ActiveRecord::Migration[7.0]
     change_column_null :anomaly_notifications, :exercise_collection_id, false
     add_foreign_key :anomaly_notifications, :exercise_collections
   end
-
-  class AnomalyNotification < ActiveRecord::Base; end
-  class Exercise < ActiveRecord::Base; end
-  class ExerciseCollection < ActiveRecord::Base; end
 end
diff --git a/db/migrate/20230821063101_convert_reason_to_json_in_anomaly_notifications.rb b/db/migrate/20230821063101_convert_reason_to_json_in_anomaly_notifications.rb
index 7718ef82..85e6e2cd 100644
--- a/db/migrate/20230821063101_convert_reason_to_json_in_anomaly_notifications.rb
+++ b/db/migrate/20230821063101_convert_reason_to_json_in_anomaly_notifications.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class ConvertReasonToJsonInAnomalyNotifications < ActiveRecord::Migration[7.0]
+  class AnomalyNotification < ApplicationRecord
+  end
+
   def up
     AnomalyNotification.where("reason LIKE '%value:%'").find_each do |anomaly_notification|
       reason = anomaly_notification.reason
@@ -14,6 +17,4 @@ class ConvertReasonToJsonInAnomalyNotifications < ActiveRecord::Migration[7.0]
   def down
     change_column :anomaly_notifications, :reason, :string
   end
-
-  class AnomalyNotification < ActiveRecord::Base; end
 end