Question

Current situation

I have two tables in my database, one for posts, and one for ratings. These are linked with a relation in the MySQL so that one post may have 0, 1 or multiple ratings, but one rating can only be applied to one post.

When I fetch a list of posts, I also want to get ratings, but without having to make a separate call to the database for each post in the foreach loop.

To do this I have attempted to use an SQL query to fetch all posts with a LEFT JOIN on ratings so that it will return a result like this:

statusId|statusBody|rating
-----------------------------
1, post1, 0
1, post1, 1
2, post2, 0
3, post3, 1
3, post3, 1

The SQL works fine, and I get the data I ask for.

Ideally what I am trying to achieve now is to turn this table into a collection of objects, with each object storing the post information as well as a value depending on it’s total ratings.

After using PDO to return the data result, this is the code I am using to map the data:

Code Logic

The logic of my code goes like this:

Get all statuses joined with ratings table

Create empty output array

Loop through PDO result
{
    Create loop specific temp array

    Push first row of result into temp array

    Remove row from PDO result

    Loop through PDO result for objects with matching statusId
    {
        If row matches statusId, add to temp buffer and remove from PDO result
    }

    Take first row of buffer and create status object

    Loop through objects in temp array to calculate ratings and add onto above status object
    Clear temp buffer

    Add status object to output array
}

return output array

Actual Code

  try 
  {
     $result = $pdo->query($sql);
     //if($result == false) return false;
     $statuses = $result->fetchAll(PDO::FETCH_CLASS, 'status');
  } 
  catch (PDOException $e) 
  {
     return FALSE;
  }

  if (!$result) {
     return FALSE;
  }

  //create empty output array to be filled up
  $status_output = array();

  //loop through all status
  foreach($statuses as $s1key => $s1value)
  {     
     //initialise temporary array;
     $status_temp_buffer = array();

     //create temp array for storing status with same ID in and add first row
     array_push($status_temp_buffer, $s1value);

     //remove from primary array
     unset($statuses[$s1key]);

     //loop through array for matching entries
     foreach($statuses as $s2key => $s2value)
     {
        //if statusId matches original, add to array;
        if($s2value->statusId == $s1value->statusId)
        {
           //add status to temp array
           array_push($status_temp_buffer, $s2value);

           //remove from primary array
           unset($statuses[$s2key]);
        }

        //stop foreach if statusId can no longer be found
        break;
     }

     //create new status object from data;
     $statObj = $status_temp_buffer[0];

     //loop through temp array to get all ratings
     foreach($status_temp_buffer as $sr)
     {
        //check if status has a rating
        if($sr->rating != NULL)
        {
           //if rating is positive...
           if($sr->rating == 1)
           {
              //add one point to positive ratings
              $statObj->totalPositiveRatings++;
           }

           //regardless add one point to total ratings
           $statObj->totalAllRatings++;
        }
     }

     //clear temporary array
     $status_temp_buffer = NULL;

     //add object to output array
     array_push($status_output, $statObj);
  }

Problem

The problem I am coming up against with this code is that although the ratings are fine, and it correctly calculates the ratings total for each post, it still shows duplicates where a post has more than one rating.

Any help with this would be greatly appreciated,
Thanks

Answer 1

As i understood it, the goal is to get the total rating of each Post entry. Instead of manually looping over each and every rating, there are two other path you could take:

• compute the total in the query:

  SELECT SUM(rating) AS total , .. FROM Posts LEFT JOIN .... GROUP BY statusID
  

  You will receive a list of Post entries, each already with total rating calculated. This is a very good solution if you have a lot of writes to to the Ratings table, and much less reads.

• the other way is to break the table normalization, but to increase read performance. What you would have to do is to add another column in the Posts table: total_rating. And have an TRIGGER on INSERT in the Ratings table, which changes the Posts.total_rating accordingly.

  This way has a benefit of simplifying the request of Posts. At the same time Ratings table can now be use to ensure that total_rating has been calculated correctly, or to recalculate the value, if there are some large changes in the ratings: like banning of user, which results in removing all ratings made by this user.