has_many :through join table conditions not applied on create

Rails 3.0.3

Given the following models

class User < ActiveRecord::Base
  has_many :roles
  has_many :owner_roles, :class_name => "Role", :conditions => {"roles.name" => "project_owner"}
  has_many :projects, :through => :roles
  has_many :owned_projects, :through => :owner_roles, :source => :project
end

class Role < ActiveRecord::Base
  belongs_to :user
  belongs_to :project
end

class Project < ActiveRecord::Base
  has_many :roles
  has_many :users, :through => :roles
end

When I attempt to use the owned_projects association to create a project for a user:

User.first.owned_projects.create

I get the error

ActiveRecord::UnknownAttributeError: unknown attribute: roles.name

The expected behavior is that the name attribute on the role record used to join the user to the new project would be set to "project_owner"