Index: app/models/principal.rb
===================================================================
--- app/models/principal.rb	(revision 17212)
+++ app/models/principal.rb	(working copy)
@@ -84,26 +84,24 @@
 
   # Principals that are members of a collection of projects
   scope :member_of, lambda {|projects|
-    projects = [projects] if projects.is_a?(Project)
-    if projects.blank?
-      where("1=0")
-    else
-      ids = projects.map(&:id)
-      active.where("#{Principal.table_name}.id IN (SELECT DISTINCT user_id FROM #{Member.table_name} WHERE project_id IN (?))", ids)
-    end
+    return where('1=0') if projects.blank?
+
+    projects = [projects] unless projects.respond_to?(:map)
+    ids = projects.map(&:id)
+    active.where("#{Principal.table_name}.id IN (SELECT DISTINCT user_id FROM #{Member.table_name} WHERE project_id IN (?))", ids)
   }
+
   # Principals that are not members of projects
   scope :not_member_of, lambda {|projects|
-    projects = [projects] unless projects.is_a?(Array)
-    if projects.empty?
-      where("1=0")
-    else
-      ids = projects.map(&:id)
-      where("#{Principal.table_name}.id NOT IN (SELECT DISTINCT user_id FROM #{Member.table_name} WHERE project_id IN (?))", ids)
-    end
+    return where('1=0') if projects.blank?
+
+    projects = [projects] unless projects.respond_to?(:map)
+    ids = projects.map(&:id)
+    where("#{Principal.table_name}.id NOT IN (SELECT DISTINCT user_id FROM #{Member.table_name} WHERE project_id IN (?))", ids)
   }
-  scope :sorted, lambda { order(*Principal.fields_for_order_statement)}
 
+  scope :sorted, lambda { order(*Principal.fields_for_order_statement) }
+
   before_create :set_default_empty_values
   before_destroy :nullify_projects_default_assigned_to
 
Index: test/unit/principal_test.rb
===================================================================
--- test/unit/principal_test.rb	(revision 17212)
+++ test/unit/principal_test.rb	(working copy)
@@ -60,7 +60,7 @@
   end
 
   def test_member_of_scope_should_be_empty_for_no_projects
-    assert_equal [], Principal.member_of([]).sort
+    assert_equal [], Principal.member_of([])
   end
 
   def test_not_member_of_scope_should_return_users_that_have_no_memberships
@@ -73,9 +73,20 @@
   end
 
   def test_not_member_of_scope_should_be_empty_for_no_projects
-    assert_equal [], Principal.not_member_of([]).sort
+    assert_equal [], Principal.not_member_of([])
   end
 
+  def test_that_not_member_of_scope_accepts_active_record_relation
+    projects_relation = Project.where(:name => 'eCookbook')
+    projects_array = projects_relation.to_a
+
+    assert_nothing_raised do
+      Principal.not_member_of(projects_relation)
+    end
+    assert_equal Principal.not_member_of(projects_array),
+                 Principal.not_member_of(projects_relation)
+  end
+
   def test_sorted_scope_should_sort_users_before_groups
     scope = Principal.where(:type => ['User', 'Group'])
     users = scope.select {|p| p.is_a?(User)}.sort