gistfile1.js

var query = new Parse.Query(Post);
query.get("xWMyZ4YEGZ", {
  success: function(post) {
    var query = new Parse.Query(Commment);
    query.equalTo("postId", "xWMyZ4YEGZ");
    query.first({
      success: function(object) {
        // Now we can finally do something
      },
      error: function(error) {
        // inner function error handling
      }
    });
  },
  error: function(object, error) {
    // outer function error handling
  }
});