About the database, you can design it like this:

CREATE TABLE "MY_OBJECT" 
(
  "ID"	INTEGER NOT NULL,
  "PROPERTY_1"	CHAR(30),
  "PROPERTY_2"	CHAR(30),
  PRIMARY KEY ("ID")
);

CREATE TABLE "EXTERNAL_ATTRIBUTE" 
(
  "ID"	INTEGER NOT NULL,
  "OBJECT_ID"	INTEGER NOT NULL,
  "NAME"	CHAR(30) NOT NULL,
  "VALUE"	CHAR(255),
 PRIMARY KEY ("ID")
);

ALTER TABLE "EXTERNAL_ATTRIBUTE" 
ADD FOREIGN KEY ("OBJECT_ID") 
REFERENCES MY_OBJECT ("ID");

your XML can be similar to this:

  prop1value
  prop2value
  
  
   ...

Each "my_object" instance is stored in a record of MY_OBJECT table, and each "external_attribute" is stored in a record of EXTERNAL_ATTRIBUTE table with an OBJECT_ID field that chain it to the referenced object.
IDs are not present in the XML because them should be managed by the application.