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GitHub User: halloffame
I know this question is really old, but I was having the same problem and realized that my rake file was saved with a .rb extension instead of .rake. Just in case anyone else has this problem in the future, this could be something to double check.
In case anyone is interested, this is a really good talk done by Yehuda Katz (one of the authors of this gem) discussing rails APIs in general and active_model_serializers specifically. He makes a lot of good arguments toward this approach, one being convention over configuration.