FIX: wasn't able to use the same username when taking over a staged account

app/models/user.rb

@@ -166,9 +166,12 @@ class User < ActiveRecord::Base
     SiteSetting.min_username_length.to_i..SiteSetting.max_username_length.to_i
   end
 
-  def self.username_available?(username)
+  def self.username_available?(username, email=nil)
     lower = username.downcase
-    !reserved_username?(lower) && !User.where(username_lower: lower).exists?
+    return false if reserved_username?(lower)
+    return true  if !User.exists?(username_lower: lower)
+    # staged users can use the same username since they will take over the account
+    email.present? && User.joins(:user_emails).exists?(staged: true, username_lower: lower, user_emails: { primary: true, email: email })
   end
 
   def self.reserved_username?(username)

app/services/username_checker_service.rb

@@ -12,7 +12,7 @@ class UsernameCheckerService
   end
 
   def check_username_availability(username, email)
-    if User.username_available?(username)
+    if User.username_available?(username, email)
       { available: true, is_developer: is_developer?(email) }
     else
       { available: false, suggestion: UserNameSuggester.suggest(username) }

spec/models/user_spec.rb

@@ -484,9 +484,16 @@ describe User do
 
     it 'returns false when a username is reserved' do
       SiteSetting.reserved_usernames = 'test|donkey'
-
       expect(User.username_available?('tESt')).to eq(false)
     end
+
+    it "returns true when username is associated to a staged user of the same email" do
+      staged = Fabricate(:user, staged: true, email: "foo@bar.com")
+      expect(User.username_available?(staged.username, staged.primary_email.email)).to eq(true)
+
+      user = Fabricate(:user, email: "bar@foo.com")
+      expect(User.username_available?(user.username, user.primary_email.email)).to eq(false)
+    end
   end
 
   describe '.reserved_username?' do