joins() on has_one through generates SQL query with "IS NULL"

I have the same problem (also using Rails 3.0.3, but using JRuby), additionally if I make the join 1 level deeper I get an INNER JOIN table_name ON 0.

class FociSample < ActiveRecord::Base
  belongs_to :foci_group
  has_one    :source_well, :through => :foci_group
end

class FociGroup < ActiveRecord::Base
  belongs_to :source_well, :foreign_key => "source_id", :class_name => "CultureWell"
end

class CultureWell < ActiveRecord::Base
  belongs_to :culture_plate
end

When trying

FociSample.joins(:source_well => :culture_plate)

the generated query is:

SELECT foci_samples.* FROM foci_samples INNER JOIN foci_groups ON foci_samples.id IS NULL INNER JOIN culture_wells ON 0 INNER JOIN culture_plates ON culture_plates.id = culture_wells.culture_plate_id

Adding an additional level

class CulturePlate < ActiveRecord::Base
  belongs_to :culture_batch
end







FociSample.joins(:source_well => {:culture_plate => :culture_batch})

results in this query:

SELECT foci_samples.* FROM foci_samples INNER JOIN foci_groups ON foci_samples.id IS NULL INNER JOIN culture_wells ON 0 INNER JOIN culture_plates ON culture_plates.id = culture_wells.culture_plate_id INNER JOIN culture_batches ON culture_batches.id = culture_plates.culture_batch_id

where the join condition for the extra dependency makes sense again.