Allow same email address with different SSO providers

config/initializers/devise.rb

@@ -12,7 +12,7 @@ Devise.setup do |config|
   # Configure the e-mail address which will be shown in Devise::Mailer,
   # note that it will be overwritten if you use your own mailer class
   # with default "from" parameter.
-  config.mailer_sender = 'please-change-me-at-config-initializers-devise@example.com'
+  config.mailer_sender = 'webmaster@screenshots.debian.net'
 
   # Configure the class responsible to send e-mails.
   # config.mailer = 'Devise::Mailer'
@@ -35,6 +35,7 @@ Devise.setup do |config|
   # You can also supply a hash where the value is a boolean determining whether
   # or not authentication should be aborted when the value is not present.
   # config.authentication_keys = [:email]
+  config.authentication_keys = [:email, :provider]
 
   # Configure parameters from the request object used for authentication. Each entry
   # given should be a request method and it will automatically be passed to the

db/migrate/20170406155141_change_users_unique_index_to_uid_provider.rb

@@ -0,0 +1,10 @@
+# A user's email address may not be unique if they use different
+# single-sign-on providers with the same email address.
+# But the combination of the email address and the (SSO)
+# provider should be unique.
+class ChangeUsersUniqueIndexToUidProvider < ActiveRecord::Migration[5.0]
+  def change
+    remove_index 'users', 'email'
+    add_index 'users', ["email", "provider"], :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: 20170405204143) do
+ActiveRecord::Schema.define(version: 20170406155141) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -78,7 +78,7 @@ ActiveRecord::Schema.define(version: 20170405204143) do
     t.datetime "locked_at"
     t.string   "provider"
     t.string   "uid"
-    t.index ["email"], name: "index_users_on_email", unique: true, using: :btree
+    t.index ["email", "provider"], name: "index_users_on_email_and_provider", unique: true, using: :btree
   end
 
   add_foreign_key "screenshots", "packages", name: "screenshots_package_id_fkey"