So my situation is: I have a 2 modules that have the same structure like that:

module Module1
  class Config

    def fee_rate
      2
    end
  end
end

So, say, Module2 would have class Config with the method fee_rate, just with a different value (those are actually implemented in a rails engine, but it shouldn't matter)

and then my model can use either Module1 or Module2 to get the fee rate value like that:

def config
  @config ||= "#{module_name.titleize}::Config".constantize.new
  @config
end

def get_value
  config.get_fee * some_other_value
end

What I'm trying to test is if get_fee function was called on the correct class:

"#{model.module_name.titleize}::Config".constantize.any_instance.expects(:get_fee).at_least_once
model.get_value

and on the line when I call get_value I get the following error - undefined method `*' for nil:NilClass. I'm completely lost now, so I'd appreciate any help and ideas.