module InheritedResources::SingletonHelpers

Singleton methods does not deal with collections.

def collection
  nil
end

:nodoc:

Returns the appropriated method to build the resource.

def method_for_association_build #:nodoc:
  :"build_#{resource_instance_name}"
end

:nodoc:

above for more information.
Sets the method_for_association_chain to nil. See resource

def method_for_association_chain #:nodoc:
  nil
end

So we have to call manager on it, not find.

Project.find(params[:project_id])

The association chain will be:

end
belongs_to :project, :singleton => true
class ManagersController < InheritedResources::Base

In singleton controllers:

chain. And this is what happened.
So you would just have to call find(:all) at the end of association

Project.find(params[:project_id]).tasks

In this case, the association chain would be:

end
belongs_to :project
class TasksController < InheritedResources::Base

end_of_association_chain here is not the same as in default belongs_to.
If you are going to overwrite it, you should notice that the

Overwrites how singleton deals with resource.

def resource
  get_resource_ivar || set_resource_ivar(end_of_association_chain.send(resource_instance_name))
end