Question

I am using a virtual field in a model. That field represents the sum of several fields: SUM(Model.amount_1 + Model.amount_2 + ...). This is the schema:

create table `current_transfers` (
  `id` integer unsigned not null auto_increment primary key,
  `description` varchar(1024) not null,
  `product_amount` decimal(13,2) default 0.0 not null,
  `tax_amount` decimal(13,2) default 0.0 not null,
  `other_amount` decimal(13,2) default 0.0 not null,
  `record_id` integer unsigned not null,
  constraint foreign key (`record_id`) references `records`(`id`) on delete cascade,
) ENGINE=InnoDB;

And this is the definition of the virtual field in CurrentTransfer model:

var $virtualFields = array(
  'amount' => 'SUM(
    CurrentTransfer.product_amount +
    CurrentTransfer.tax_amount +
    CurrentTransfer.other_amount)'
);

Let’s say the table does not have any records yet and you use $this->paginate() in RecordsController to retrieve a list of results, like this:

class RecordsController extends AppController {

  var $name = 'Records';

  var $paginate = array(
    'CurrentTransfer' => array(
      'order' => array('CurrentTransfer.amount' => 'desc'),
    ),
  );

  // ...

  public function view($id) {
    // ...
    $this->paginate('CurrentTransfer', array('CurrentTransfer.record_id' => $id));
    // ...
  }

  // ...
}

I have found two different behaviors:

• If the virtual field is actually defined in the model the result is an array with one element storing an empty record, i.e. every field is set to NULL.
• If the virtual field is not defined in the model the result is an empty array, which is the common behavior in CakePHP when a query returns an empty set of values.

I would always expect the second behavior, since is easier to compare if there are results or not.

Why is CakePHP returning a fictitious result when there are actually no records in the table? Why the addition of virtual fields produce such behavior?

Answer 1

SUM() is an aggregate function that sums up all rows in a column, not all columns in a row as you probably want. As an aggregate function, it always returns a value, at least NULL. That means the SQL query returns at least one row with all columns set to NULL. Cake is simply picking this up.

I think you’re actually looking for this:

public $virtualFields = array(
  'amount' => 'CurrentTransfer.product_amount + CurrentTransfer.tax_amount + CurrentTransfer.other_amount'
);

I.e. the simple + operator without the SUM().