Actually the changeset doesn't need an id attribute in the xml. It is simply fetched as a parameter in the url. Thanks for pointing it out Frederik. We really need more tests so that things like this are thought about more before committing potentially significant changes.

app/models/changeset.rb

@@ -74,12 +74,6 @@ class Changeset < ActiveRecord::Base
       cs.closed_at = cs.created_at + IDLE_TIMEOUT
       # initially we have no changes in a changeset
       cs.num_changes = 0
-    else
-      raise OSM::APIBadXMLError.new("changeset", pt, "ID is required when updating.") if pt['id'].nil?
-      cs.id = pt['id'].to_i
-      # .to_i will return 0 if there is no number that can be parsed.
-      # We want to make sure that there is no id with zero anyway.
-      raise OSM::APIBadUserInput.new("ID of changeset cannot be zero when updating.") if cs.id == 0
     end
 
     pt.find('tag').each do |tag|
@@ -106,9 +100,9 @@ class Changeset < ActiveRecord::Base
   # returns area of the changset bbox as a rough comparitive quantity for use of changset displays
   def area
      if has_valid_bbox?
-	     (max_lon - min_lon) * (max_lat - min_lat)
+       (max_lon - min_lon) * (max_lat - min_lat)
      else
-	     0
+       0
      end
   end
 

test/unit/changeset_test.rb

@@ -7,20 +7,6 @@ class ChangesetTest < ActiveSupport::TestCase
     assert_equal 7, Changeset.count
   end
 
-  def test_xml_from_id_zero
-    id_list = ["", "0", "00", "0.0", "a"]
-    id_list.each do |id|
-      zero_id = "<osm><changeset id='#{id}' /></osm>"
-      assert_nothing_raised(OSM::APIBadUserInput) {
-        Changeset.from_xml(zero_id, true)
-      }
-      message_update = assert_raise(OSM::APIBadUserInput) {
-        Changeset.from_xml(zero_id, false)
-      }
-      assert_match /ID of changeset cannot be zero when updating/, message_update.message
-    end
-  end
-  
   def test_from_xml_no_text
     no_text = ""
     message_create = assert_raise(OSM::APIBadXMLError) {
@@ -58,7 +44,7 @@ class ChangesetTest < ActiveSupport::TestCase
   end
 
   def test_from_xml_no_v
-    no_v = "<osm><changeset id='1'><tag k='key' /></changeset></osm>"
+    no_v = "<osm><changeset><tag k='key' /></changeset></osm>"
     message_create = assert_raise(OSM::APIBadXMLError) {
       Changeset.from_xml(no_v, true)
     }
@@ -70,7 +56,7 @@ class ChangesetTest < ActiveSupport::TestCase
   end
 
   def test_from_xml_duplicate_k
-    dupk = "<osm><changeset id='1'><tag k='dup' v='test' /><tag k='dup' v='value' /></changeset></osm>"
+    dupk = "<osm><changeset><tag k='dup' v='test' /><tag k='dup' v='value' /></changeset></osm>"
     message_create = assert_raise(OSM::APIDuplicateTagsError) {
       Changeset.from_xml(dupk, true)
     }
@@ -78,6 +64,17 @@ class ChangesetTest < ActiveSupport::TestCase
     message_update = assert_raise(OSM::APIDuplicateTagsError) {
       Changeset.from_xml(dupk, false)
     }
-    assert_equal "Element changeset/1 has duplicate tags with key dup", message_update.message
+    assert_equal "Element changeset/ has duplicate tags with key dup", message_update.message
+  end
+
+  def test_from_xml_valid
+    # Example taken from the Update section on the API_v0.6 docs on the wiki
+    xml = "<osm><changeset><tag k=\"comment\" v=\"Just adding some streetnames and a restaurant\"/></changeset></osm>"
+    assert_nothing_raised(OSM::APIBadXMLError) {
+      Changeset.from_xml(xml, false)
+    }
+    assert_nothing_raised(OSM::APIBadXMLError) {
+      Changeset.from_xml(xml, true)
+    }
   end
 end