Add event model

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