add unique index on Users#email

db/migrate/20211219074758_index_users_by_unique_email.rb

@@ -0,0 +1,5 @@
+class IndexUsersByUniqueEmail < ActiveRecord::Migration[6.1]
+  def change
+    add_index :users, :email, unique: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_08_11_115732) do
+ActiveRecord::Schema.define(version: 2021_12_19_074758) do
 
   create_table "articles", charset: "utf8", force: :cascade do |t|
     t.string "name", null: false
@@ -76,6 +76,7 @@ ActiveRecord::Schema.define(version: 2019_08_11_115732) do
     t.datetime "created_at"
     t.datetime "updated_at"
     t.boolean "admin", default: false
+    t.index ["email"], name: "index_users_on_email", unique: true
   end
 
 end