diff --git a/Gemfile b/Gemfile
index 0f2f3bf..93e3b88 100644
--- a/Gemfile
+++ b/Gemfile
@@ -37,6 +37,9 @@ gem 'devise-encryptable', '0.2.0'
 # Reduces boot times through caching; required in config/boot.rb
 gem 'bootsnap', '>= 1.4.2', require: false
 
+# for composite primary keys
+gem 'composite_primary_keys'
+
 group :development, :test do
   # Call 'byebug' anywhere in the code to stop execution and get a debugger console
   gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
diff --git a/app/controllers/crawlers/projects_crawler_controller.rb b/app/controllers/crawlers/projects_crawler_controller.rb
index 460f496..c25cb31 100644
--- a/app/controllers/crawlers/projects_crawler_controller.rb
+++ b/app/controllers/crawlers/projects_crawler_controller.rb
@@ -36,9 +36,9 @@ class Crawlers::ProjectsCrawlerController < Crawlers::TissCrawlerController
     @project = TissCrawler.get_project_details(params)
 
     # create stores the object to the db after creation
-    favorite_hash = {id: params[:id],
+    favorite_hash = {id: [params[:id], current_user.id],
                      # the user who is currently active
-                     user_id: current_user.id,
+                     # user_id: current_user.id,
                      title: @project['titleDe']}
     if FavoriteProject.create(favorite_hash).valid?
       FavoriteProject.create(favorite_hash)
diff --git a/app/controllers/crawlers/theses_crawler_controller.rb b/app/controllers/crawlers/theses_crawler_controller.rb
index ae2d336..97c8e65 100644
--- a/app/controllers/crawlers/theses_crawler_controller.rb
+++ b/app/controllers/crawlers/theses_crawler_controller.rb
@@ -36,9 +36,9 @@ class Crawlers::ThesesCrawlerController < Crawlers::TissCrawlerController
     @thesis = TissCrawler.get_thesis_details(params)
 
     # create stores the object to the db after creation
-    favorite_hash = {id: params[:id],
+    favorite_hash = {id: [params[:id], current_user.id],
                      # the user who is currently active
-                     user_id: current_user.id,
+                     # user_id: current_user.id,
                      title: @thesis['title']['de']}
     if FavoriteThesis.create(favorite_hash).valid?
       FavoriteThesis.create(favorite_hash)
diff --git a/app/models/favorite_course.rb b/app/models/favorite_course.rb
index 60d0d1f..7bac3e6 100644
--- a/app/models/favorite_course.rb
+++ b/app/models/favorite_course.rb
@@ -1,4 +1,6 @@
 class FavoriteCourse < ApplicationRecord
+  self.primary_keys = :id, :user_id
+
   validates :title, :semester, :number, presence: true
-  validates :semester, :number, uniqueness: {scope: [:semester, :number]}
+  validates :semester, :number, uniqueness: {scope: [:semester, :number, :user_id]}
 end
diff --git a/app/models/favorite_person.rb b/app/models/favorite_person.rb
index 9d1e8b8..e5d4186 100644
--- a/app/models/favorite_person.rb
+++ b/app/models/favorite_person.rb
@@ -1,5 +1,5 @@
 class FavoritePerson < ApplicationRecord
-  self.primary_key = 'tiss_id'
+  self.primary_keys = :tiss_id, :user_id
 
-  validates :tiss_id, uniqueness: true
+  validates :tiss_id, uniqueness: { scope: [:user_id] }
 end
diff --git a/app/models/favorite_project.rb b/app/models/favorite_project.rb
index f7482c4..ca4e37e 100644
--- a/app/models/favorite_project.rb
+++ b/app/models/favorite_project.rb
@@ -1,5 +1,5 @@
 class FavoriteProject < ApplicationRecord
-  self.primary_key = 'id'
+  self.primary_keys = :id, :user_id
 
-  validates :id, uniqueness: true
+  validates :id, :user_id, uniqueness: {scope: [:id, :user_id]}
 end
diff --git a/app/models/favorite_thesis.rb b/app/models/favorite_thesis.rb
index f1618f9..23f5238 100644
--- a/app/models/favorite_thesis.rb
+++ b/app/models/favorite_thesis.rb
@@ -1,5 +1,5 @@
 class FavoriteThesis < ApplicationRecord
-  self.primary_key = 'id'
+  self.primary_keys = :id, :user_id
 
-  validates :id, uniqueness: true
+  validates :id, :user_id, uniqueness: {scope: [:id, :user_id]}
 end
diff --git a/db/migrate/20200421152314_create_favorite_people.rb b/db/migrate/20200421152314_create_favorite_people.rb
index 039d27d..e412e40 100644
--- a/db/migrate/20200421152314_create_favorite_people.rb
+++ b/db/migrate/20200421152314_create_favorite_people.rb
@@ -1,15 +1,13 @@
 class CreateFavoritePeople < ActiveRecord::Migration[6.0]
   def change
-    create_table :favorite_people, id: false do |t|
+    create_table :favorite_people, primary_key: [:tiss_id, :user_id] do |t|
       t.integer :tiss_id, null: false
-      t.integer :user_id, null: false
+      t.belongs_to :user
       t.string :first_name
       t.string :last_name
       t.string :picture_uri
       t.string :personal_annotation
-
       t.timestamps
     end
-    add_index :favorite_people, :tiss_id, unique: true
   end
 end
diff --git a/db/migrate/20200521091231_create_favorite_courses.rb b/db/migrate/20200521091231_create_favorite_courses.rb
index d475a0b..45b675e 100644
--- a/db/migrate/20200521091231_create_favorite_courses.rb
+++ b/db/migrate/20200521091231_create_favorite_courses.rb
@@ -1,14 +1,14 @@
 class CreateFavoriteCourses < ActiveRecord::Migration[6.0]
   def change
-    create_table :favorite_courses do |t|
+    create_table :favorite_courses , primary_key: [:id, :user_id] do |t|
+      t.integer :id
+      t.belongs_to :user
       t.string :number, null: false
       t.string :semester, null: false
       t.string :title, null: false
-      t.integer :user_id, null: false
       t.string :personal_annotation
 
       t.timestamps
     end
-    add_index :favorite_courses, [:number, :semester], unique: true
   end
 end
diff --git a/db/migrate/20200525132942_create_favorite_theses.rb b/db/migrate/20200525132942_create_favorite_theses.rb
index c0edcef..4292946 100644
--- a/db/migrate/20200525132942_create_favorite_theses.rb
+++ b/db/migrate/20200525132942_create_favorite_theses.rb
@@ -1,13 +1,12 @@
 class CreateFavoriteTheses < ActiveRecord::Migration[6.0]
   def change
-    create_table :favorite_theses, id: false do |t|
+    create_table :favorite_theses, primary_key: [:id, :user_id] do |t|
       t.integer :id, null: false
-      t.integer :user_id, null: false
+      t.belongs_to :user
       t.string :title
       t.string :personal_annotation
 
       t.timestamps
     end
-    add_index :favorite_theses, :id, unique: true
   end
 end
diff --git a/db/migrate/20200529081537_create_favorite_projects.rb b/db/migrate/20200529081537_create_favorite_projects.rb
index 701e4e5..90dc787 100644
--- a/db/migrate/20200529081537_create_favorite_projects.rb
+++ b/db/migrate/20200529081537_create_favorite_projects.rb
@@ -1,13 +1,12 @@
 class CreateFavoriteProjects < ActiveRecord::Migration[6.0]
   def change
-    create_table :favorite_projects, id: false do |t|
+    create_table :favorite_projects, primary_key: [:id, :user_id] do |t|
       t.integer :id, null: false
-      t.integer :user_id, null: false
+      t.belongs_to :user
       t.string :title
       t.string :personal_annotation
 
       t.timestamps
     end
-    add_index :favorite_projects, :id, unique: true
   end
 end
diff --git a/db/schema.rb b/db/schema.rb
index 603affc..88fe6f3 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -12,47 +12,48 @@
 
 ActiveRecord::Schema.define(version: 2020_05_29_081537) do
 
-  create_table "favorite_courses", force: :cascade do |t|
+  create_table "favorite_courses", primary_key: ["id", "user_id"], force: :cascade do |t|
+    t.integer "id"
+    t.integer "user_id"
     t.string "number", null: false
     t.string "semester", null: false
     t.string "title", null: false
-    t.integer "user_id", null: false
     t.string "personal_annotation"
     t.datetime "created_at", precision: 6, null: false
     t.datetime "updated_at", precision: 6, null: false
-    t.index ["number", "semester"], name: "index_favorite_courses_on_number_and_semester", unique: true
+    t.index ["user_id"], name: "index_favorite_courses_on_user_id"
   end
 
-  create_table "favorite_people", id: false, force: :cascade do |t|
+  create_table "favorite_people", primary_key: ["tiss_id", "user_id"], force: :cascade do |t|
     t.integer "tiss_id", null: false
-    t.integer "user_id", null: false
+    t.integer "user_id"
     t.string "first_name"
     t.string "last_name"
     t.string "picture_uri"
     t.string "personal_annotation"
     t.datetime "created_at", precision: 6, null: false
     t.datetime "updated_at", precision: 6, null: false
-    t.index ["tiss_id"], name: "index_favorite_people_on_tiss_id", unique: true
+    t.index ["user_id"], name: "index_favorite_people_on_user_id"
   end
 
-  create_table "favorite_projects", id: false, force: :cascade do |t|
+  create_table "favorite_projects", primary_key: ["id", "user_id"], force: :cascade do |t|
     t.integer "id", null: false
-    t.integer "user_id", null: false
+    t.integer "user_id"
     t.string "title"
     t.string "personal_annotation"
     t.datetime "created_at", precision: 6, null: false
     t.datetime "updated_at", precision: 6, null: false
-    t.index ["id"], name: "index_favorite_projects_on_id", unique: true
+    t.index ["user_id"], name: "index_favorite_projects_on_user_id"
   end
 
-  create_table "favorite_theses", id: false, force: :cascade do |t|
+  create_table "favorite_theses", primary_key: ["id", "user_id"], force: :cascade do |t|
     t.integer "id", null: false
-    t.integer "user_id", null: false
+    t.integer "user_id"
     t.string "title"
     t.string "personal_annotation"
     t.datetime "created_at", precision: 6, null: false
     t.datetime "updated_at", precision: 6, null: false
-    t.index ["id"], name: "index_favorite_theses_on_id", unique: true
+    t.index ["user_id"], name: "index_favorite_theses_on_user_id"
   end
 
   create_table "users", force: :cascade do |t|