add index to tables

db/migrate/20191126082849_create_users.rb

@@ -6,7 +6,7 @@ class CreateUsers < ActiveRecord::Migration[6.0]
 
       ## Database authenticatable
       t.string :email, null: false, default: ""
-      t.string :username
+      t.string :username, unique: true
       t.string :encrypted_password, null: false, default: ""
 
       ## Recoverable

db/migrate/20191126082917_create_companies.rb

@@ -8,5 +8,7 @@ class CreateCompanies < ActiveRecord::Migration[6.0]
 
       t.timestamps
     end
+
+    add_index :companies, :email, unique: true
   end
 end

db/migrate/20191126082933_create_cities.rb

@@ -6,5 +6,7 @@ class CreateCities < ActiveRecord::Migration[6.0]
 
       t.timestamps
     end
+    
+    add_index :cities, :name, unique: true
   end
 end