Add StudyGroups with ExternalUsers only

app/models/external_user.rb

@@ -4,11 +4,11 @@ class ExternalUser < User
   validates :external_id, presence: true
 
   def displayname
-    result = name
-    if(result == nil || result == "")
-      result = "User " + id.to_s
+    if name.blank?
+      "User " + id.to_s
+    else
+      name
     end
-    result
   end
 
 end

app/models/study_group.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class StudyGroup < ApplicationRecord
+  has_many :study_group_memberships
+  # Use `ExternalUser` as `source_type` for now.
+  # Using `User` will lead ActiveRecord to access the inexistent table `users`.
+  # Issue created: https://github.com/rails/rails/issues/34531
+  has_many :users, through: :study_group_memberships, source_type: 'ExternalUser'
+  has_many :submissions
+  belongs_to :consumer
+end

app/models/study_group_membership.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class StudyGroupMembership < ApplicationRecord
+  belongs_to :user, polymorphic: true
+  belongs_to :study_group
+
+  validates_uniqueness_of :user_id, :scope => [:user_type, :study_group_id]
+end

app/models/submission.rb

@@ -6,6 +6,7 @@ class Submission < ApplicationRecord
   FILENAME_URL_PLACEHOLDER = '{filename}'
 
   belongs_to :exercise
+  belongs_to :study_group, optional: true
 
   has_many :testruns
   has_many :structured_errors

app/models/user.rb

@@ -4,6 +4,8 @@ class User < ApplicationRecord
   ROLES = %w(admin teacher)
 
   belongs_to :consumer
+  has_many :study_group_memberships, as: :user
+  has_many :study_groups, through: :study_group_memberships, as: :user
   has_many :exercises, as: :user
   has_many :file_types, as: :user
   has_many :submissions, as: :user