Aggregate Fields

An example model

Say you want to model a bank account and all their entries. Entries into the account can either be of positive or negative money values. Each account has a credit limit and the account is never allowed to have a balance below that value.

For simplicity we live in a world where money is composed of integers only. Also we omit the receiver/sender name, stated reason for transfer and the execution date. These all would have to be added on the Entry object.

Our entities look like:

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<?php

namespace Bank\Entities;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;

#[ORM\Entity]
class Account
{
    #[ORM\Id]
    #[ORM\GeneratedValue]
    #[ORM\Column(type: 'integer')]
    private ?int $id;

    #[ORM\OneToMany(targetEntity: Entry::class, mappedBy: 'account', cascade: ['persist'])]
    private Collection $entries;


    public function __construct(
        #[ORM\Column(type: 'string', unique: true)]
        private string $no,

        #[ORM\Column(type: 'integer')]
        private int $maxCredit = 0,
    ) {
        $this->entries = new ArrayCollection();
    }
}

#[ORM\Entity]
class Entry
{
    #[ORM\Id]
    #[ORM\GeneratedValue]
    #[ORM\Column(type: 'integer')]
    private ?int $id;

    public function __construct(
        #[ORM\ManyToOne(targetEntity: Account::class, inversedBy: 'entries')]
        private Account $account,

        #[ORM\Column(type: 'integer')]
        private int $amount,
    ) {
        // more stuff here, from/to whom, stated reason, execution date and such
    }

    public function getAmount(): Amount
    {
        return $this->amount;
    }
}
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Using DQL

The Doctrine Query Language allows you to select for aggregate values computed from fields of your Domain Model. You can select the current balance of your account by calling:

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<?php
$dql = "SELECT SUM(e.amount) AS balance FROM Bank\Entities\Entry e " .
       "WHERE e.account = ?1";
$balance = $em->createQuery($dql)
              ->setParameter(1, $myAccountId)
              ->getSingleScalarResult();
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The $em variable in this (and forthcoming) example holds the Doctrine EntityManager. We create a query for the SUM of all amounts (negative amounts are withdraws) and retrieve them as a single scalar result, essentially return only the first column of the first row.

This approach is simple and powerful, however it has a serious drawback. We have to execute a specific query for the balance whenever we need it.

To implement a powerful domain model we would rather have access to the balance from our Account entity during all times (even if the Account was not persisted in the database before!).

Also an additional requirement is the max credit per Account rule.

We cannot reliably enforce this rule in our Account entity with the DQL retrieval of the balance. There are many different ways to retrieve accounts. We cannot guarantee that we can execute the aggregation query for all these use-cases, let alone that a userland programmer checks this balance against newly added entries.

Using your Domain Model

Account and all the Entry instances are connected through a collection, which means we can compute this value at runtime:

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<?php
class Account
{
    // .. previous code

    public function getBalance(): int
    {
        $balance = 0;
        foreach ($this->entries as $entry) {
            $balance += $entry->getAmount();
        }

        return $balance;
    }
}
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Now we can always call Account::getBalance() to access the current account balance.

To enforce the max credit rule we have to implement the "Aggregate Root" pattern as described in Eric Evans book on Domain Driven Design. Described with one sentence, an aggregate root controls the instance creation, access and manipulation of its children.

In our case we want to enforce that new entries can only added to the Account by using a designated method. The Account is the aggregate root of this relation. We can also enforce the correctness of the bi-directional Account <-> Entry relation with this method:

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<?php
class Account
{
    public function addEntry(int $amount): void
    {
        $this->assertAcceptEntryAllowed($amount);

        $this->entries[] = new Entry($this, $amount);
    }
}
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Now look at the following test-code for our entities:

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<?php

use PHPUnit\Framework\TestCase;

class AccountTest extends TestCase
{
    public function testAddEntry()
    {
        $account = new Account("123456", maxCredit: 200);
        $this->assertEquals(0, $account->getBalance());

        $account->addEntry(500);
        $this->assertEquals(500, $account->getBalance());

        $account->addEntry(-700);
        $this->assertEquals(-200, $account->getBalance());
    }

    public function testExceedMaxLimit()
    {
        $account = new Account("123456", maxCredit: 200);

        $this->expectException(Exception::class);
        $account->addEntry(-1000);
    }
}
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To enforce our rule we can now implement the assertion in Account::addEntry:

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<?php

class Account
{
    // .. previous code

    private function assertAcceptEntryAllowed(int $amount): void
    {
        $futureBalance = $this->getBalance() + $amount;
        $allowedMinimalBalance = ($this->maxCredit * -1);
        if ($futureBalance < $allowedMinimalBalance) {
            throw new Exception("Credit Limit exceeded, entry is not allowed!");
        }
    }
}
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We haven't talked to the entity manager for persistence of our account example before. You can call EntityManager::persist($account) and then EntityManager::flush() at any point to save the account to the database. All the nested Entry objects are automatically flushed to the database also.

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<?php
$account = new Account("123456", 200);
$account->addEntry(500);
$account->addEntry(-200);
$em->persist($account);
$em->flush();
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The current implementation has a considerable drawback. To get the balance, we have to initialize the complete Account::$entries collection, possibly a very large one. This can considerably hurt the performance of your application.

Using an Aggregate Field

To overcome the previously mentioned issue (initializing the whole entries collection) we want to add an aggregate field called "balance" on the Account and adjust the code in Account::getBalance() and Account:addEntry():

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<?php
class Account
{
    #[ORM\Column(type: 'integer')]
    private int $balance = 0;

    public function getBalance(): int
    {
        return $this->balance;
    }

    public function addEntry(int $amount): void
    {
        $this->assertAcceptEntryAllowed($amount);

        $this->entries[] = new Entry($this, $amount);
        $this->balance += $amount;
    }
}
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This is a very simple change, but all the tests still pass. Our account entities return the correct balance. Now calling the Account::getBalance() method will not occur the overhead of loading all entries anymore. Adding a new Entry to the Account::$entities will also not initialize the collection internally.

Adding a new entry is therefore very performant and explicitly hooked into the domain model. It will only update the account with the current balance and insert the new entry into the database.

Tackling Race Conditions with Aggregate Fields

Whenever you denormalize your database schema race-conditions can potentially lead to inconsistent state. See this example:

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<?php

use Bank\Entities\Account;

// The Account $accId has a balance of 0 and a max credit limit of 200:
// request 1 account
$account1 = $em->find(Account::class, $accId);

// request 2 account
$account2 = $em->find(Account::class, $accId);

$account1->addEntry(-200);
$account2->addEntry(-200);

// now request 1 and 2 both flush the changes.
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The aggregate field Account::$balance is now -200, however the SUM over all entries amounts yields -400. A violation of our max credit rule.

You can use both optimistic or pessimistic locking to safe-guard your aggregate fields against this kind of race-conditions. Reading Eric Evans DDD carefully he mentions that the "Aggregate Root" (Account in our example) needs a locking mechanism.

Optimistic locking is as easy as adding a version column:

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<?php

class Account
{
    #[ORM\Column(type: 'integer')]
    #[ORM\Version]
    private int $version;
}
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The previous example would then throw an exception in the face of whatever request saves the entity last (and would create the inconsistent state).

Pessimistic locking requires an additional flag set on the EntityManager::find() call, enabling write locking directly in the database using a FOR UPDATE.

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<?php

use Bank\Entities\Account;
use Doctrine\DBAL\LockMode;

$account = $em->find(Account::class, $accId, LockMode::PESSIMISTIC_WRITE);
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Keeping Updates and Deletes in Sync

The example shown in this article does not allow changes to the value in Entry, which considerably simplifies the effort to keep Account::$balance in sync. If your use-case allows fields to be updated or related entities to be removed you have to encapsulate this logic in your "Aggregate Root" entity and adjust the aggregate field accordingly.

Conclusion

This article described how to obtain aggregate values using DQL or your domain model. It showed how you can easily add an aggregate field that offers serious performance benefits over iterating all the related objects that make up an aggregate value. Finally I showed how you can ensure that your aggregate fields do not get out of sync due to race-conditions and concurrent access.