class ActiveRecord::AmbiguousSourceReflectionForThroughAssociation

def initialize(klass, macro, association_name, options, possible_sources)
  example_options = options.dup
  example_options[:source] = possible_sources.first
  super("Ambiguous source reflection for through association. Please " \
        "specify a :source directive on your declaration like:\n" \
        "\n" \
        "  class #{klass} < ActiveRecord::Base\n" \
        "    #{macro} :#{association_name}, #{example_options}\n" \
        "  end"
       )
end