DB can have PK factory. Collection#insert returns objects inserted (possibly modified by PK factory).

README.rdoc

@@ -94,8 +94,10 @@ Here is a sample primary key factory, taken from the tests:
     end
   end
 
-A database's PK factory object may not be changed. Right now, it may only be
-specified when you obtain the db object.
+A database's PK factory object may be changed, but this is not recommended.
+The only reason it is changeable is so that libraries such as MongoRecord that
+use this driver can set the PK factory after obtaining the database but before
+using it for the first time.
 
 === Strict mode
 

lib/mongo/collection.rb

@@ -68,6 +68,9 @@ module XGen
         end
 
         # Insert +objects+, which are hashes. "<<" is aliased to this method.
+        # Returns either the single inserted object or a new array containing
+        # +objects+. The object(s) may have been modified by the database's PK
+        # factory, if it has one.
         def insert(*objects)
           objects = objects.first if objects.size == 1 && objects.first.is_a?(Array)
           res = @db.insert_into_db(@name, objects)

lib/mongo/db.rb

@@ -63,6 +63,11 @@ module XGen
         # DB#new for details.
         attr_reader :pk_factory
 
+        def pk_factory=(pk_factory)
+          raise "error: can not change PK factory" if @pk_factory
+          @pk_factory = pk_factory
+        end
+
         # db_name :: The database name
         #
         # nodes :: An array of [host, port] pairs.
@@ -345,12 +350,14 @@ module XGen
         end
 
         # Insert +objects+ into +collection_name+. Normally called by
-        # Collection#insert.
+        # Collection#insert. Returns a new array containing +objects+,
+        # possibly modified by @pk_factory.
         def insert_into_db(collection_name, objects)
           @semaphore.synchronize {
-            objects.each { |o|
+            objects.collect { |o|
               o = @pk_factory.create_pk(o) if @pk_factory
               send_to_db(InsertMessage.new(@name, collection_name, o))
+              o
             }
           }
         end

tests/test_db.rb

@@ -60,20 +60,26 @@ class DBTest < Test::Unit::TestCase
     coll = db.collection('test')
     coll.clear
 
-    coll.insert('name' => 'Fred')
+    obj = coll.insert('name' => 'Fred', 'age' => 42)
     row = coll.find({'name' => 'Fred'}, :limit => 1).next_object
-    assert_not_nil row
-    assert_equal 'Fred', row['name']
-    assert_kind_of XGen::Mongo::Driver::ObjectID, row['_id']
+    assert_equal obj, row
 
     oid = XGen::Mongo::Driver::ObjectID.new
-    coll.insert('_id' => oid, 'name' => 'Barney')
+    obj = coll.insert('_id' => oid, 'name' => 'Barney', 'age' => 41)
     row = coll.find({'name' => 'Barney'}, :limit => 1).next_object
-    assert_not_nil row
-    assert_equal 'Barney', row['name']
-    assert_equal oid, row['_id']
+    assert_equal obj, row
 
     coll.clear
   end
 
+  def test_pk_factory_reset
+    @db.pk_factory = Object.new # first time
+    begin
+      @db.pk_factory = Object.new
+      fail "error: expected exception"
+    rescue => ex
+      assert_match /can not change PK factory/, ex.to_s
+    end
+  end
+
 end