Add Moped::BSON.ObjectId(string)

- For compatibility with apps that may use the 10gen driver API with
  regards to creating object ids from strings.

- No inspection override here.

- Closes mongoid#4.

lib/moped/bson.rb

@@ -21,5 +21,22 @@ module BSON
     FLOAT_PACK = 'E'.freeze
 
     START_LENGTH = [0].pack(INT32_PACK).freeze
+
+    class << self
+
+      # Create a new object id from the provided string.
+      #
+      # @example Create a new object id.
+      #   Moped::BSON::ObjectId("4faf83c7dbf89b7b29000001")
+      #
+      # @param [ String ] string The string to use.
+      #
+      # @return [ ObjectId ] The object id.
+      #
+      # @since 1.0.0
+      def ObjectId(string)
+        ObjectId.from_string(string)
+      end
+    end
   end
 end

spec/moped/bson_spec.rb

@@ -0,0 +1,52 @@
+require "spec_helper"
+
+describe Moped::BSON do
+
+  describe ".ObjectId" do
+
+    context "when provided a string" do
+
+      context "when the string is a valid id" do
+
+        let(:id) do
+          described_class.ObjectId("4faf83c7dbf89b7b29000001")
+        end
+
+        let(:expected) do
+          Moped::BSON::ObjectId.from_string("4faf83c7dbf89b7b29000001")
+        end
+
+        it "returns an object id" do
+          id.should eq(expected)
+        end
+      end
+
+      context "when the string is not a valid id" do
+
+        it "raises an error" do
+          expect {
+            described_class.ObjectId("test")
+          }.to raise_error(Moped::Errors::InvalidObjectId)
+        end
+      end
+    end
+
+    context "when provided a non string" do
+
+      it "raises an error" do
+        expect {
+          described_class.ObjectId(1)
+        }.to raise_error
+      end
+    end
+
+    context "when provided nil" do
+
+      it "raises an error" do
+        expect {
+          described_class.ObjectId(nil)
+        }.to raise_error
+      end
+    end
+  end
+end