Add event model

app/models/event.rb

@@ -0,0 +1,5 @@
+class Event < ActiveRecord::Base
+  belongs_to :user, polymorphic: true
+  belongs_to :exercise
+  belongs_to :file
+end

db/migrate/20180814145059_create_events.rb

@@ -0,0 +1,12 @@
+class CreateEvents < ActiveRecord::Migration
+  def change
+    create_table :events do |t|
+      t.string :type
+      t.string :data
+      t.belongs_to :user, polymorphic: true, index: true
+      t.belongs_to :exercise, index: true
+      t.belongs_to :file, index: true
+      t.timestamps null: false
+    end
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180703125302) do
+ActiveRecord::Schema.define(version: 20180814145059) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -95,6 +95,21 @@ ActiveRecord::Schema.define(version: 20180703125302) do
 
   add_index "errors", ["submission_id"], name: "index_errors_on_submission_id", using: :btree
 
+  create_table "events", force: :cascade do |t|
+    t.string   "type"
+    t.string   "data"
+    t.integer  "user_id"
+    t.string   "user_type"
+    t.integer  "exercise_id"
+    t.integer  "file_id"
+    t.datetime "created_at",  null: false
+    t.datetime "updated_at",  null: false
+  end
+
+  add_index "events", ["exercise_id"], name: "index_events_on_exercise_id", using: :btree
+  add_index "events", ["file_id"], name: "index_events_on_file_id", using: :btree
+  add_index "events", ["user_type", "user_id"], name: "index_events_on_user_type_and_user_id", using: :btree
+
   create_table "execution_environments", force: :cascade do |t|
     t.string   "docker_image",             limit: 255
     t.string   "name",                     limit: 255