diff --git a/lib/ubiquo_media/media_selector/active_record.rb b/lib/ubiquo_media/media_selector/active_record.rb
index 040502c..f5fd5d6 100644
--- a/lib/ubiquo_media/media_selector/active_record.rb
+++ b/lib/ubiquo_media/media_selector/active_record.rb
@@ -89,9 +89,19 @@ module UbiquoMedia

           # WARNING: asset_types are cached since you define media_attachment on a model!
           # A server restart could be required when adding new asset_types
-          options[:types] = AssetType.get_by_keys(options[:types])
+          # It's a lambda to be initialized on demand and not on boot time, to avoid
+          # some issues with empty asset_type table on tests.
+          initialize_types = lambda do
+            unless options[:types].is_a?( Array ) && options[:types].first.is_a?( AssetType )
+              options[:types] = AssetType.get_by_keys(options[:types])
+            end
+          end

           proc = Proc.new do
+            def initialize
+              initialize_types.call
+              super
+            end

             def is_full?
               return false if self.options[:size].to_sym == :many
@@ -180,6 +190,7 @@ module UbiquoMedia
           end

           define_method "valid_asset_types_in_#{field}" do
+            initialize_types.call
             invalid = send("#{field}_current_asset_relations").to_a.detect do |asset|
               # current_asset_relations can return assets or asset_relations
               asset = asset.respond_to?( :asset ) ? asset.asset : asset