MySQL ON UPDATE CASCADE not CASCADEing

Suppose i have two below table:

CREATE TABLE post (
  id bigint(20)     NOT NULL    AUTO_INCREMENT,
  text text ,

  PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=1;

CREATE TABLE post_path (
  ancestorid bigint(20)     NOT NULL    DEFAULT '0',
  descendantid bigint(20)   NOT NULL    DEFAULT '0',
  length int(11)            NOT NULL    DEFAULT '0',

  PRIMARY KEY (ancestorid,descendantid),
  KEY descendantid (descendantid),

  CONSTRAINT f_post_path_ibfk_1 
    FOREIGN KEY (ancestorid) REFERENCES post (id) 
    ON DELETE CASCADE 
    ON UPDATE CASCADE,
  CONSTRAINT f_post_path_ibfk_2 
    FOREIGN KEY (descendantid) REFERENCES post (id) 
    ON DELETE CASCADE 
    ON UPDATE CASCADE
) ENGINE=InnoDB;

And inserted these rows:

INSERT INTO 
    post (text)
    VALUES ('a'); #// inserted row by id=1
INSERT INTO 
    post_path (ancestorid ,descendantid ,length) 
    VALUES (1, 1, 0);

When i want to update post row id:

 UPDATE post SET id = '10' WHERE post.id =1

MySQL said:

#1452 - Cannot add or update a child row: a foreign key constraint fails (test.post_path, CONSTRAINT f_post_path_ibfk_2 FOREIGN KEY (descendantid) REFERENCES post (id) ON DELETE CASCADE ON UPDATE CASCADE) 

Why? what is wrong?

Edit:

When i inserted these rows:

INSERT INTO 
    post (text)
    VALUES ('b'); #// inserted row by id=2

INSERT INTO 
    post_path (ancestorid, descendantid, length)
    VALUES (1, 2, 0);

And updated:

UPDATE post SET id = '20' WHERE post.id =2

Mysql updated successfully both child and parent row. so Why i can not update first post (id=1)?