Problem with custom "_id" in "before.insert()" of "collection hook"

help
1

Problem with custom “_id” in “before.insert()”.

// fixture
var locatData = {........}; // don't have _id field
var locatId = Location.insert(locatData);
console.log('id after insert: ' + locatId);

// hook
Location.before.insert(function (userId, doc) {
    doc._id = '0001';
});

// Result
id after insert: random id
// It should be '0001'

Please help me.