Through Associations with Conditions Fails

Suppose you have a class as such:

class Earning
  belongs_to :earning_category,
    :conditions => 'department_code = \'#{department_code_for_instance}\''
end

And another like this:

class Employee
  has_many :earnings, :include => :earning_category
end

You will have an error like the following when attempting to access the 'has_many :earnings' association on an Employee instance:

NoMethodError: undefined method `department_code' for #<ActiveRecord::Associations::ClassMethods::JoinDependency::JoinAssociation:0x685c5cc>
        from /vendor/rails/activerecord/lib/active_record/associations.rb:2143:in `interpolate_sql'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:2104:in `association_join'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:2103:in `each'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:2103:in `association_join'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:1566:in `construct_finder_sql_with_included_associations'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:1566:in `collect'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:1566:in `construct_finder_sql_with_included_associations'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:1558:in `select_all_rows'
        from /vendor/rails/activerecord/lib/active_record/associations.rb:1335:in `find_with_associations'
from C:/projects/409a/vendor/rails/activerecord/lib/active_record/base.rb:1551:in
`find_every'
        from /vendor/rails/activerecord/lib/active_record/base.rb:615:in `find'
        from /vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:60:in `find'
        from /vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:395:in `find_target'
        from /vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:349:in `load_target'
        from /vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:139:in `inspect'

It seems to me that the interpolation of the belongs_to conditions is happening in the wrong context.