Let identifier_field be customizable, analgous to timestamp_field

To be used for a feature I actually need in the ActiveRecordWrapper, and will implement there next.

Sorry, haven't figured out how to test this at present. The customizable timestamp_field does not seem to be tested either.

Author: jrochkind

lib/oai/provider.rb

@@ -155,8 +155,10 @@
 #
 # Valid options include:
 #
-# * `timestamp_field` - Specifies the model field to use as the update
+# * `timestamp_field` - Specifies the model field/method to use as the update
 #   filter.  Defaults to `updated_at`.
+# * `identifier_field` -- specifies the model field/method to use to get value to use
+#    as oai identifier (method return value should not include prefix)
 # * `limit` -           Maximum number of records to return in each page/set.
 #   Defaults to 100, set to `nil` for all records in one page. Otherwise
 #   the wrapper will paginate the result via resumption tokens.

lib/oai/provider/model.rb

@@ -29,10 +29,11 @@ module OAI::Provider
   # see the ResumptionToken class for more details.
   #
   class Model
-    attr_reader :timestamp_field
+    attr_reader :timestamp_field, :identifier_field
 
-    def initialize(limit = nil, timestamp_field = 'updated_at')
+    def initialize(limit = nil, timestamp_field = 'updated_at', identifier_field = 'id')
       @limit = limit
+      @identifier_field = identifier_field
       @timestamp_field = timestamp_field
     end
 

lib/oai/provider/response/record_response.rb

@@ -50,7 +50,7 @@ def about_for(record)
 
     # Namespace syntax suggested in http://www.openarchives.org/OAI/2.0/guidelines-oai-identifier.htm
     def identifier_for(record)
-      "#{provider.prefix}:#{record.id}"
+      "#{provider.prefix}:#{record.send( provider.model.identifier_field )}"
     end
 
     def timestamp_for(record)