Yes, it is.
Rails defines pluralization rules within ActiveSupport. You can see the rules defined here: github.com/rails/rails/blob/master/activesupport/…
The plural form of "matrix" is defined as "matrices". You can verify this by running "Matrix".tableize" in Rails console which will return "matrices" .
I'm not completely certain about this, but "Matrix" may actually be a reserved class name in Rails as there exists a ruby class called Matrix.