Error: When using mongo gem in RoR:

Notice: The native BSON extension was not loaded. For optimal performance, use of the BSON extension is recommended. To enable the extension make sure ENV[‘BSON_EXT_DISABLED’] is not set and run the following command: gem install bson_ext

Cause:

bson_ext and mongo gem are not in the same version.

Solution: Install bson_ext and mongo gems using the same version, example: 1.9.2:

gem install mongo -v=1.9.2
gem install bson_ext -v=1.9.2