Defining a custom to_json method for your classes can be useful. Do it properly or you will "randomly" get errors like these:

wrong number of arguments (1 for 0) (ArgumentError)
wrong number of arguments (0 for 1) (ArgumentError)

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Take a look at how Object#to_json is defined:

def to_json(options = nil)
  ActiveSupport::JSON.encode(as_json(options))
end

Make sure you at least take the options argument -- or, if you don't need to look at it, just grab and (if you need to) pass on any arguments you receive like this:

def to_json(*args, &block)
  my_custom_representation.to_json(*args, &block)
end

That way, outside code can call your to_json method any way they like.