list tables and columns of non-polymorphic foreign keys

fk.rb

ApplicationRecord.descendants.flat_map do |c|
  c.reflect_on_all_associations.select do |a|
    ActiveRecord::Reflection::BelongsToReflection === a && !a.options[:polymorphic]
  end
end.map do |r|
  [r.active_record.table_name, r.plural_name, r.options.slice(:optional, :foreign_key)]
end.uniq.sort

Also used the optional option to determine if the fk column should be not null.