If you defined your association via

class Article
  belongs_to "category"
end

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and you try

Article.scoped(:include => :category)

you will get an error message

in `preload_one_association': Association named 'category' was not found; perhaps you misspelled it? (ActiveRecord::ConfigurationError)

Solution

Always define your assocations via symbol

class Article
  belongs_to :category
end