Question

I made a simple following-follower system with php(pdo) and mysql. My problem is,

Let’s say there is a user name Mike with ID number 99. And Mike has two followers, Greg(id = 77) and Jenny(id = 88)

Greg’s following list looks like this (1,2,3,4,99), and Jenny’s following list looks like this (5,6,99,7)

What I am trying to do is, when Mike(99) deletes his account, I want to remove id number 99 from Greg(77) and Jenny(88)’s following lists.

Table called ‘mytable’ has 3 fields. id(INT), following_list(text), follower_list(text).

I’ve been struggling with this problem for a few days. Can anyone please give me some advice? Thank you so much in advance!!!

Below is my update function

public function update_following_list(){
   // $this->myid is Mike's id number(99) who is about to delete his account
   // Selecting the list of people following Mike
   $query = "SELECT 'follower_list' FROM 'mytable' WHERE 'id' = $this->myid";
   $result = $this->dbc->query($query);
      foreach($result as $row){
      $this->follower_list = $row['follower_list'];
      // When I echo $this->follower_list, I get 77,88

      // Now querying Greg(77) and Jenny(88)'s following_lists, which have Mike's id number(99) in them.
      $query = "SELECT 'following_list' FROM 'mytable' WHERE 'id' IN ($this->follower_list)";
      $result = $this->dbc->query($query);
         foreach($result as $row){
         $this->following_list = $row['following_list'];
         // When I echo $this->following_list, I get both Greg(1,2,3,4,99) and Jenny(5,6,99,7)'s following lists

         // Here, I am turning following list into array by using explode
         $this->following_array = explode(",", $this->following_list);
            foreach($this->following_array as $key => $value){
            if($value == $this->myid){

                // Removing Mike(99)'s id number from Greg and Jenny's following lists
                unset($this->following_array[$key]);

                // Add back commas, which will then become string
                $this->new_following_list = implode(",", $this->_following_array);
                // When I echo $this->new_following_list, I get both Greg(1,2,3,4) and Jenny(5,6,7)'s new list without Mike(99)'s id number 

// My problem starts here. I was able to remove Mike's id number from Greg and Jenny's lists. But I am having a trouble updating Greg and Jenny's following lists with new following lists.  

            // The update query below does not work...
            $query = "UPDATE 'mytable' SET 'following_list' = $this->new_following_list WHERE 'id' IN ($this->follower_list)";
            $result = $this->dbc->query($query);
        } // End of if($value == $this->myid)
        } // End of foreach($this->following_array as $key => $value)
    }
    } 
} // End of function

Answer 1

This will not scale properly. It’s better to normalize your data model like this:

following

user_id | following_user_id
---------------------------
     77 |                 1
     77 |                 2
     77 |                 3
     77 |                 4
     77 |                99
     88 |                 5
     88 |                 6
     88 |                 7
     88 |                99

And add two indexes:

• UNIQUE(user_id, following_user_id)
• INDEX(following_user_id)

To get followers of 99:

SELECT * FROM following WHERE following_user_id=99;

To see who 77 follows:

SELECT * FROM following WHERE user_id=77;