arangosh> db.test.ensureUniqueConstraint("a", "b.c");
{ 
  "id" : "test/651974044", 
  "type" : "hash", 
  "fields" : [ 
    "a", 
    "b.c" 
  ], 
  "selectivityEstimate" : 1, 
  "unique" : true, 
  "sparse" : false, 
  "isNewlyCreated" : true, 
  "code" : 201 
}
arangosh> db.test.save({ a : 1, b : { c : 1 } });
{ 
  "_id" : "test/652301724", 
  "_rev" : "652301724", 
  "_key" : "652301724" 
}
arangosh> db.test.save({ a : 1, b : { c : 1 } });
[ArangoError 1210: cannot create document, unique constraint violated]
arangosh> db.test.save({ a : 1, b : { c : null } });
{ 
  "_id" : "test/652760476", 
  "_rev" : "652760476", 
  "_key" : "652760476" 
}
arangosh> db.test.save({ a : 1 });
[ArangoError 1210: cannot create document, unique constraint violated]