尝试自动将外键值插入我的mysql表

Question

I am new with PHP and MySQL and need help...

Im cant seem to get my head around how to automatically insert a foreign key into a table in my database. The primary key gets added as 'id' in my 'mem' table but I want this same id to be added to a foreign key in my 'location' table. Can anyone please help?

I have two tables...

'mem table' id, username, email.

  |    1    |    2    |    3    |
  |   paul  |   john  | francis |       
  | paul@gm.| john@gm.|francis@.|

'location table' id, location, user_id << forign key

  |    1    |    2    |    3    |
  |  leeds  |liverpool| london  |       
  |    ?    |    ?    |    ?    | 

how do I get the 'id' in the user table to automatically get inserted here as a foreign key?

Im using this php code to insert the data...

$sql = mysql_query("INSERT INTO mem (username, country, state, city, accounttype, email, password, signupdate) 
        VALUES('$username','$country','$state','$city','$accounttype','$email','$hashedPass', now())")      or die(mysql_error());
$id = mysql_insert_id();

$sql = mysql_query("INSERT INTO location (location) 
VALUES('$location')") or die(mysql_error());
$user_id = mysql_insert_id();

Im getting this error when I submit the form... Cannot add or update a child row: a foreign key constraint fails ( mem . location , CONSTRAINT location_ibfk_1 FOREIGN KEY ( user_id ) REFERENCES location ( id ) ON DELETE CASCADE ON UPDATE CASCADE)

Can anyone help?

Answer 1

your FK is in the wrong direction. In the moment it says: "Every entry in user table MUST have a entry in location table." You try to insert in the user table, without creating a location entry first. So the FK fails.

solution: Set the FK on the location table "user_id -> users.user_id" so it means "every user_id on the location table should have an entry in the users table.". The only Problem is: the "ON DELETE CASCADE" is no longer working, when you delete a entry in the users table... - This can be solved with a database trigger (PSEUDOCODE) "BEFORE delete FROM users DELETE FROM locations WHERE user_id" or you just do 2 queries in PHP. (which is the easier solution. 1st delete location row, 2nd the user row.)