#550 ✓not-applicable
Ward Asbridge

Models and class inheritance oddness

Reported by Ward Asbridge | September 4th, 2008 @ 04:10 PM

While experimenting with base model classes, I've found that DM doesn't seem to handle child models which extend from base classes. This scenario is the closest I got it to working - calling "rake dm:db:automigrate" causes an SQL error, as DM tries to create an empty media_bases table. If I add a dummy field to the base class, such as "property :id, Serial", then DM does create a dummy media_bases table, but doesn't create a media_videos table at all.


class Media
  class Base

    include DataMapper::Resource

    #property :id, Serial

    def inherited(model)
      model.class_eval do
        property :id,             Serial
        property :title,          String,   :length => 255
        property :description,    Text
        property :created_at,     DateTime
        property :updated_at,     DateTime
        property :deleted_at,     DateTime
        property :created_user,   String, :length => 255
        property :updated_user,   String, :length => 255
        property :deleted_user,   String, :length => 255
        property :filesize,       Integer



require 'media'
class Media
  class Video < Media::Base

    include DataMapper::Resource

    property :runtime,        Integer
    property :aspect,         String


Comments and changes to this ticket

  • Dan Kubb (dkubb)

    Dan Kubb (dkubb) January 8th, 2009 @ 02:20 PM

    • State changed from “new” to “unconfirmed”
    • Assigned user cleared.
  • Dan Kubb (dkubb)

    Dan Kubb (dkubb) February 21st, 2009 @ 01:38 AM

    • Tag set to dm-core
    • State changed from “unconfirmed” to “not-applicable”

    The usage of a true inherited method would likely cause a bit of problems because DM::Resource uses its own to handle inheritance. However, in this case, you've defined it as an instance method so it probably would have had no effect.

    The key to getting this to work is to add a discriminator field, like this:

      property :type, Discriminator

    DM only really works with inheritance when using a discriminator property. While this may change at some point in the future, it is how it is currently designed to work.

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