MySQL - Cannot add or update a child row: a foreign key constraint fails

I had faced same issue while creating foreign constraints on table. the simple way of coming out of this issue are first take backup of your parent and child table then truncate child table and again try to make a relation. hope this will solve the problem.

Since you haven't given table definitions, it's hard to guess. But it looks like you are attempting to modify the foreign key in the child table. AFAIK, this is illegal, you can modify it from the parent, but not the child table.

Consider this example:

CREATE TABLE parent (
  parent_id INT NOT NULL,
  parent_data int,

  PRIMARY KEY (parent_id)
) ENGINE=INNODB;

CREATE TABLE child1 (
  child1_id INT,
  child1_data INT,
  fk_parent_id INT,

  INDEX par_ind1 (fk_parent_id),

  FOREIGN KEY (fk_parent_id)
    REFERENCES parent(parent_id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
) ENGINE=INNODB;

CREATE TABLE child2 (
  child2_id INT,
  child2_data INT,
  fk_parent_id INT,

  INDEX par_ind2 (fk_parent_id),

  FOREIGN KEY (fk_parent_id)
    REFERENCES parent(parent_id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
) ENGINE=INNODB;

INSERT INTO parent
  (parent_id, parent_data)
  VALUES
  (1, 11),
  (2, 12);

INSERT INTO child1
  (child1_id, child1_data, fk_parent_id)
  VALUES
  (101, 1001, 1),
  (102, 1002, 1),
  (103, 1003, 1),
  (104, 1004, 2),
  (105, 1005, 2);

INSERT INTO child2
  (child2_id, child2_data, fk_parent_id)
  VALUES
  (106, 1006, 1),
  (107, 1007, 1),
  (108, 1008, 1),
  (109, 1009, 2),
  (110, 1010, 2);

Then this is allowed:

UPDATE parent
  SET parent_id = 3 WHERE parent_id = 2;

SELECT * FROM parent;
SELECT * FROM child1;
SELECT * FROM child2;

But this is not, because it modifies the parent fk from the child table:

UPDATE child1
  SET fk_parent_id = 4 WHERE fk_parent_id = 1;

It gets an error very similar to your error:

Cannot add or update a child row: a foreign key constraint fails (`db_2_b43a7`.`child1`, CONSTRAINT `child1_ibfk_1` FOREIGN KEY (`fk_parent_id`) REFERENCES `parent` (`parent_id`) ON DELETE CASCADE ON UPDATE CASCADE):

I solved my 'foreign key constraint fails' issues by adding the following code to the start of the SQL code (this was for importing values to a table)

SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT;
SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS;
SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION;
SET NAMES utf8;
SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0;
SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0;
SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO';
SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0; 

Then adding this code to the end of the file

SET SQL_MODE=@OLD_SQL_MODE;
SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS;
SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS;
SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT;
SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS;
SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION;
SET SQL_NOTES=@OLD_SQL_NOTES;