Question

I need to extend an existing module (namely Redmine::MimeType) by adding a few items to a constant MIME_TYPES Hash.

This is what I tried so far and that only gives me a “dynamic constant assignment” error:

module MimeTypePatch
  def self.included(base) # :nodoc:
    base.extend(ClassMethods)

    base.class_eval do
      MIME_TYPES_VIDEO = {
        'video/x-flv' => 'flv,f4v',
        'video/mpeg' => '*.mpeg *.mpg *.mpe',
        'video/quicktime' => 'qt,mov',
        'video/vnd.vivo' => 'viv,vivo',
        'video/x-msvideo' => 'avi'
      }.freeze

      # merge the new mime types with the existing ones
      MIME_TYPES = MIME_TYPES.merge(MIME_TYPES_VIDEO).freeze

      EXTENSIONS = MIME_TYPES.inject({}) do |map, (type, exts)|
        exts.split(',').each {|ext| map[ext.strip] = type}
        map
      end
    end
  end
end

So what am I doing wrong and what might be the correct approach to alter an existing constant in another module? I do see that changing a constant at runtime is a bit off, but I couldn’t think of a more elegant approach to achieve what I want (detecting video mime types).

Answer 1

you can’t assign a value to constant after initial definition, you have to create a new constant which will have the merged hash value. Change this line from

# merge the new mime types with the existing ones
MIME_TYPES = MIME_TYPES.merge(MIME_TYPES_VIDEO).freeze

to

# merge the new mime types with the existing ones
MERGED_MIME_TYPES = MIME_TYPES.merge(MIME_TYPES_VIDEO).freeze

and then you can use this new constant MERGED_MIME_TYPES in further logic