Cannot fnd bson_ext in path

I am getting the highly poular message:

** 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

If you continue to receive this message after installing, make sure that
the bson_ext gem is in your load path.

I am running rbenv and have tried gem install bson_ext. I get no errors - it tells me Successfully installed bson_ext-1.12.3. However bson_ext is not found - I assume its an executable. How do I know where it should be installed so that I can update my path accordingly?

If I search in my local .rbenv dir I see:

.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/bson_ext-1.12.3/ext/bson_ext/bson_ext/

But do not see anything that appears to be an executable.

via Chebli Mohamed