Ruby on Rails: How to check if the Model Exists

Since defined? is problematic (see @Jiggneshh Gohel's comment), perhaps you can check the filenames in the models dir.

files = Dir[Rails.root + 'app/models/*.rb']
models = files.map{ |m| File.basename(m, '.rb').camelize }

models.include? "User" => true

defined? ModelName will return "constant" if model defined.