diff --git a/README.rdoc b/README.rdoc
index 7e67d81..ac0e121 100644
--- a/README.rdoc
+++ b/README.rdoc
@@ -63,7 +63,58 @@ read from Mongo will have their character encodings set to UTF-8.
 
 When used with Ruby 1.8, the bytes in each string are written to and read from
 Mongo as-is. If the string is ASCII all is well, because ASCII is a subset of
-UTF-8. If the string is not ASCII then it may not be a well-formed UTF-8 string.
+UTF-8. If the string is not ASCII then it may not be a well-formed UTF-8
+string.
+
+== The DB class
+
+=== Primary Key Factories
+
+A basic Mongo driver is not responsible for creating primary keys or knowing
+how to interpret them. You can tell the Ruby Mongo driver how to create
+primary keys by passing in the :pk option to the Mongo#db method.
+
+  include XGen::Mongo::Driver
+  db = Mongo.new.db('dbname', :pk => MyPKFactory)
+
+A primary key factory object must respond to :create_pk, which should take a
+hash and return a hash which merges the original hash with any primary key
+fields the factory wishes to inject. NOTE: if the object already has a primary
+key, the factory should not inject a new key; this means that the object is
+being used in a repsert but it already exists. The idea here is that when ever
+a record is inserted, the :pk object's +create_pk+ method will be called and
+the new hash returned will be inserted.
+
+Here is a sample primary key factory, taken from the tests:
+
+  class TestPKFactory
+    def create_pk(row)
+      row['_id'] ||= XGen::Mongo::Driver::ObjectID.new
+      row
+    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.
+
+=== Strict mode
+
+Each database has an optional strict mode. If strict mode is on, then asking
+for a collection that does not exist will raise an error, as will asking to
+create a collection that already exists. Note that both these operations are
+completely harmless; strict mode is a programmer convenience only.
+
+To turn on strict mode, either pass in :strict => true when obtaining a DB
+object or call the :strict= method:
+
+  db = XGen::Mongo::Driver::Mongo.new.db('dbname', :strict => true)
+  # I'm feeling lax
+  db.strict = false
+  # No, I'm not!
+  db.strict = true
+
+The method DB#strict? returns the current value of that flag.
+
 
 = Testing
 
diff --git a/lib/mongo/db.rb b/lib/mongo/db.rb
index 84a5d9e..e581904 100644
--- a/lib/mongo/db.rb
+++ b/lib/mongo/db.rb
@@ -59,17 +59,40 @@ module XGen
         # The database's socket. For internal (and Cursor) use only.
         attr_reader :socket
 
+        # A primary key factory object (or +nil+). See the README.doc file or
+        # DB#new for details.
+        attr_reader :pk_factory
+
         # db_name :: The database name
         #
         # nodes :: An array of [host, port] pairs.
         #
+        # options :: A hash of options.
+        #
+        # Options:
+        #
+        # :strict :: If true, collections must exist to be accessed and must
+        #            not exist to be created. See #collection and
+        #            #create_collection.
+        #
+        # :pk :: A primary key factory object that must respond to :create_pk,
+        #        which should take a hash and return a hash which merges the
+        #        original hash with any primary key fields the factory wishes
+        #        to inject. (NOTE: if the object already has a primary key,
+        #        the factory should not inject a new key; this means that the
+        #        object is being used in a repsert but it already exists.) The
+        #        idea here is that when ever a record is inserted, the :pk
+        #        object's +create_pk+ method will be called and the new hash
+        #        returned will be inserted.
+        #
         # When a DB object first connects, it tries the first node. If that
         # fails, it keeps trying to connect to the remaining nodes until it
         # sucessfully connects.
-        def initialize(db_name, nodes)
+        def initialize(db_name, nodes, options={})
           raise "Invalid DB name" if !db_name || (db_name && db_name.length > 0 && db_name.include?("."))
           @name, @nodes = db_name, nodes
-          @strict = false
+          @strict = options[:strict]
+          @pk_factory = options[:pk]
           @semaphore = Object.new
           @semaphore.extend Mutex_m
           connect_to_first_available_host
@@ -252,8 +275,8 @@ module XGen
         # applying +obj+ as an update. If no match, inserts (???). Normally
         # called by Collection#repsert.
         def repsert_in_db(collection_name, selector, obj)
-          # TODO if PKInjector, inject
           @semaphore.synchronize {
+            obj = @pk_factory.create_pk(obj) if @pk_factory
             send_to_db(UpdateMessage.new(@name, collection_name, selector, obj, true))
             obj
           }
@@ -325,7 +348,10 @@ module XGen
         # Collection#insert.
         def insert_into_db(collection_name, objects)
           @semaphore.synchronize {
-            objects.each { |o| send_to_db(InsertMessage.new(@name, collection_name, o)) }
+            objects.each { |o|
+              o = @pk_factory.create_pk(o) if @pk_factory
+              send_to_db(InsertMessage.new(@name, collection_name, o))
+            }
           }
         end
 
diff --git a/lib/mongo/mongo.rb b/lib/mongo/mongo.rb
index 3b948dc..0d57cf3 100644
--- a/lib/mongo/mongo.rb
+++ b/lib/mongo/mongo.rb
@@ -52,9 +52,10 @@ module XGen
                    end
         end
 
-        # Return the XGen::Mongo::Driver::DB named +db_name+.
-        def db(db_name)
-          XGen::Mongo::Driver::DB.new(db_name, @nodes)
+        # Return the XGen::Mongo::Driver::DB named +db_name+. See DB#new for
+        # +options+.
+        def db(db_name, options={})
+          XGen::Mongo::Driver::DB.new(db_name, @nodes, options)
         end
 
         # Returns a hash containing database names as keys and disk space for
diff --git a/tests/test_db.rb b/tests/test_db.rb
index e7ef926..adac947 100644
--- a/tests/test_db.rb
+++ b/tests/test_db.rb
@@ -2,6 +2,13 @@ $LOAD_PATH[0,0] = File.join(File.dirname(__FILE__), '..', 'lib')
 require 'mongo'
 require 'test/unit'
 
+class TestPKFactory
+  def create_pk(row)
+    row['_id'] ||= XGen::Mongo::Driver::ObjectID.new
+    row
+  end
+end
+
 # NOTE: assumes Mongo is running
 class DBTest < Test::Unit::TestCase
 
@@ -48,4 +55,25 @@ class DBTest < Test::Unit::TestCase
     assert @db.connected?
   end
 
+  def test_pk_factory
+    db = Mongo.new(@host, @port).db('ruby-mongo-test', :pk => TestPKFactory.new)
+    coll = db.collection('test')
+    coll.clear
+
+    coll.insert('name' => 'Fred')
+    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']
+
+    oid = XGen::Mongo::Driver::ObjectID.new
+    coll.insert('_id' => oid, 'name' => 'Barney')
+    row = coll.find({'name' => 'Barney'}, :limit => 1).next_object
+    assert_not_nil row
+    assert_equal 'Barney', row['name']
+    assert_equal oid, row['_id']
+
+    coll.clear
+  end
+
 end