> db.foo.insert({a:{b:"hello"}})
> db.foo.ensureIndex({"a.b":"text"})
> db.foo.validate().keysPerIndex
{ "test.foo.$_id_" : 1, "test.foo.$a.b_text" : 1 } // correct: 1 entry in text index
> db.foo.dropIndexes()
{
	"nIndexesWas" : 2,
	"msg" : "non-_id indexes dropped for collection",
	"ok" : 1
}
> db.foo.ensureIndex({"a.b":"text", a:"text"}) // now index "a" in addition
> db.foo.validate().keysPerIndex
{ "test.foo.$_id_" : 1, "test.foo.$a.b_text_a_text" : 0 } // incorrect: 0 entries in text index