> db.foo.ensureIndex({a:"text"})
WriteResult({ "nInserted" : 1 })
> db.foo.insert({a:"hello world"})
WriteResult({ "nInserted" : 1 })
> db.foo.find({$text:{$search:"hello"}}) // works before creating {_fts:1} index
{ "_id" : ObjectId("53151b403d6c9988adb3eed8"), "a" : "hello world" }
> db.foo.ensureIndex({_fts:1})
WriteResult({ "nInserted" : 1 })
> db.foo.find({$text:{$search:"hello"}}) // fails after creating {_fts:1} index
error: {
	"$err" : "Unable to execute query: error processing query: ns=test.foo limit=0 skip=0\nTree: TEXT : query=hello, language = , tag=First: 0 1 notFirst: full path: _fts\nSort: {}\nProj: {}\n planner returned error: need exactly one text index for $text query",
	"code" : 17007
}
>