gistfile1.rb

class Keygroup
	include DataMapper::Resource

	has n, :keywords, :through => Resource
	has n, :categories, :through => Resource

	property :id, Serial
	property :name, String
	property :description, Text
	property :created_at, DateTime
	property :updated_at, DateTime

	after :create, :create_master_keyword

	def create_master_keyword
		key = Keyword.new
		key.name = self.name
		key.master = true
		key.keygroups << self
		raise RuntimeError, "Keygroup's Master Keyword has not been saved!" unless key.save
	end

end

class Keyword
	include DataMapper::Resource

	has n, :keygroups, :through => Resource
	has n, :images, :through => Resource
	has 1, :store
	has 1, :example

	property :id, Serial
	property :name, String
	property :master, Boolean, :default => false
	property :created_at, DateTime, :writer => :private
	property :updated_at, DateTime, :writer => :private
	
	def master?
		self.master
	end

end

grp = Keygroup.get(1)
wrd = Keyword.get(1)
grp.keywords << wrd
grp.save # returns false
grp.errors.each{|e| puts e } # empty

It turns out that the error was not in the grp object but the wrd as would be revealed by doing a wrd.inspect