dado que el :unassigned puede partir del scope :alljoins, puedes encadenarlos:

scope :alljoins ,joins: "left join projects on projects.admin_id==employees.id left join teams on...
scope :recent, lambda { alljoins.conditions: "" }

http://www.slashdotdash.net/2010/09/25/rails-3-scopes-with-chaining/