dm_association_bug.rb

require 'test/unit/assertions'
require 'dm-core'
require 'dm-sqlite-adapter'
require 'dm-migrations'
 
DataMapper.setup :default, "sqlite::memory:"
 
class Tag
  include DataMapper::Resource

  property :id, Serial
  property :category, String

  has n, :listing_tags
  has n, :tags, :through => :listing_tags
end

class ListingTag
  include DataMapper::Resource

  property :id, Serial

  belongs_to :listing
  belongs_to :tag
end

class Listing
  include DataMapper::Resource

  property :id, Serial

  has n, :listing_tags
  has n, :tags, :through => :listing_tags
end

DataMapper.auto_migrate!

listing = Listing.create
listing.tags.create(category: "niche")

listing.listing_tags.reload

include Test::Unit::Assertions

# this passes, the id of the tag matches the tag_id of the listing_tag
assert_equal listing.tags.last.id, listing.listing_tags.last.tag_id

listing.listing_tags.tags(category: "other")

# this fails, the tag_id of the listing_tag is now nil
assert_equal listing.tags.last.id, listing.listing_tags.last.tag_id

I'm curious, should https://gist.github.com/lsimoneau/4591743#file-dm_association_bug-rb-L15 be has n :listings …?