form_errors_for_associations.rb

module ActiveModelInstanceTagWithAssociations

  def error_message
    Rails.logger.debug "ActiveModelInstanceTagWithAssociations: " + @method_name.inspect

    if @method_name.end_with?('_ids')
      # Check for a has_(and_belongs_to_)many association (these always use the _ids postfix field).
      association = object.class.reflect_on_association(@method_name.chomp('_ids').pluralize.to_sym)
    else
      # Check for a belongs_to association with method_name matching the foreign key column
      association = object.class.reflect_on_all_associations.find do |a|
        a.macro == :belongs_to && a.foreign_key == @method_name
      end
    end
    if association.present?
      object.errors[association.name] + super
    else
      super
    end
  end

end

ActionView::Helpers::ActiveModelInstanceTag.prepend(ActiveModelInstanceTagWithAssociations)

Modified from code posted here but I can't get it to work under Rails 5.1.