Rails - Nov 2007 - ActiveRecord::Migration is a mix-in?

I''m going by <http://railsforum.com/viewtopic.php?id=1011> and am
curious
about this line:

class AddUserIdToPosts < ActiveRecord::Migration


AddUserIdToPosts is a class, ok.  ActiveRecord is a module, ok.  
Migration is a class?  This is an example of a mix-in?

I don''t really understand this line :(


-Thufir


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