Gab Social. All are welcome.

app/validators/unique_username_validator.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class UniqueUsernameValidator < ActiveModel::Validator
+  def validate(account)
+    return if account.username.nil?
+
+    normalized_username = account.username.downcase.delete('.')
+
+    scope = Account.where(domain: nil).where('lower(username) = ?', normalized_username)
+    scope = scope.where.not(id: account.id) if account.persisted?
+
+    account.errors.add(:username, :taken) if scope.exists?
+  end
+end