Never ever declare your associations with a string, especially when doing metaprogramming. A common mistake is something like

# WRONG
class Page < ActiveRecord::Base
  %w[main sub].each do |type|
    belongs_to "#{type}_title"
  end
end

   
# RIGHT
class Page < ActiveRecord::Base
  %w[main sub].each do |type|
    belongs_to :"#{type}_title"
  end
end

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Always convert to a symbol, otherwise you'll have all kinds of fun.

Newer rails / activerecord (at least 5.2) will throw an association names must be a Symbol (ArgumentError) error if you define an association as string.