Association named 'variations' was not found; perhaps you misspelled it?

I just was hunting down a strange error with this model:

class Model
  placeholder = 'variations'
  
  has_many placeholder
  
  nested_scope :verbose, :include => :variations
end

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Everything, including Model.variations and Model.reflect_on_all_associations.map &:name did work (the latter returned [ :some_association, 'variations' ]).
Eventually, the quotes led me to the solution: I had to write has_many placeholder.to_sym.

When Rails can't find an association, make sure it's defined as symbol

Seemingly, that's a bug in Rails.