318 lines
10 KiB
Plaintext
318 lines
10 KiB
Plaintext
# Aggregate Fields
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You will often come across the requirement to display aggregate values of data that
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can be computed by using the MIN, MAX, COUNT or SUM SQL functions. For any ORM this is a tricky issue traditionally. Doctrine 2 offers several ways to get access to these values and this article will describe all of them from different perspectives.
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You will see that aggregate fields can become very explicit
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features in your domain model and how this potentially complex business rules
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can be easily tested.
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## An example model
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Say you want to model a bank account and all their entries. Entries
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into the account can either be of positive or negative money values.
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Each account has a credit limit and the account is never allowed
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to have a balance below that value.
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For simplicity we live in a world were money is composed of integers
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only. Also we omit the receiver/sender name, stated reason for transfer
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and the execution date. These all would have to be added on the `Entry`
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object.
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Our entities look like:
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[php]
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namespace Bank\Entities;
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/**
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* @Entity
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*/
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class Account
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{
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/** @Id @GeneratedValue @Column(type="integer") */
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private $id;
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/** @Column(type="string", unique=true) */
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private $no;
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/**
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* @OneToMany(targetEntity="Entry", mappedBy="entries", cascade={"persist"})
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*/
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private $entries;
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/**
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* @Column(type="integer")
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*/
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private $maxCredit = 0;
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public function __construct($no, $maxCredit = 0)
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{
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$this->no = $no;
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$this->maxCredit = $maxCredit;
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$this->entries = new \Doctrine\Common\Collections\ArrayCollection();
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}
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}
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/**
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* @Entity
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*/
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class Entry
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{
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/** @Id @GeneratedValue @Column(type="integer") */
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private $id;
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/**
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* @ManyToOne(targetEntity="Account", inversedBy="entries")
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*/
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private $account;
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/**
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* @Column(type="integer")
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*/
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private $amount;
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public function __construct($account, $amount)
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{
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$this->account = $account;
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$this->amount = $amount;
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// more stuff here, from/to whom, stated reason, execution date and such
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}
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public function getAmount()
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{
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return $this->amount;
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}
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}
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## Using DQL
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The Doctrine Query Language allows you to select for aggregate values computed from
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fields of your Domain Model. You can select the current balance of your account by
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calling:
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[php]
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$dql = "SELECT SUM(e.amount) AS balance FROM Bank\Entities\Entry e " .
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"WHERE e.account = ?1";
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$balance = $em->createQuery($dql)
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->setParameter(1, $myAccountId)
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->getSingleScalarResult();
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The `$em` variable in this (and forthcoming) example holds the Doctrine `EntityManager`.
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We create a query for the SUM of all amounts (negative amounts are withdraws) and
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retrieve them as a single scalar result, essentially return only the first column
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of the first row.
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This approach is simple and powerful, however it has a serious drawback. We have
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to execute a specific query for the balance whenever we need it.
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To implement a powerful domain model we would rather have access to the balance from
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our `Account` entity during all times (even if the Account was not persisted
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in the database before!).
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Also an additional requirement is the max credit per `Account` rule.
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We cannot reliably enforce this rule in our `Account` entity with the DQL retrieval
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of the balance. There are many different ways to retrieve accounts. We cannot
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guarantee that we can execute the aggregation query for all these use-cases,
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let alone that a userland programmer checks this balance against newly added
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entries.
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## Using your Domain Model
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`Account` and all the `Entry` instances are connected through a collection,
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which means we can compute this value at runtime:
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[php]
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class Account
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{
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// .. previous code
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public function getBalance()
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{
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$balance = 0;
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foreach ($this->entries AS $entry) {
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$balance += $entry->getAmount();
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}
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return $balance;
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}
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}
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Now we can always call `Account::getBalance()` to access the current account balance.
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To enforce the max credit rule we have to implement the "Aggregate Root" pattern as
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described in Eric Evans book on Domain Driven Design. Described with one sentence,
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an aggregate root controls the instance creation, access and manipulation of its children.
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In our case we want to enforce that new entries can only added to the `Account` by
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using a designated method. The `Account` is the aggregate root of this relation.
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We can also enforce the correctness of the bi-directional `Account` <-> `Entry`
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relation with this method:
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[php]
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class Account
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{
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public function addEntry($amount)
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{
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$this->assertAcceptEntryAllowed($amount);
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$e = new Entry($this, $amount);
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$this->entries[] = $e;
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return $e;
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}
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}
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Now look at the following test-code for our entities:
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[php]
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class AccountTest extends \PHPUnit_Framework_TestCase
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{
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public function testAddEntry()
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{
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$account = new Account("123456", $maxCredit = 200);
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$this->assertEquals(0, $account->getBalance());
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$account->addEntry(500);
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$this->assertEquals(500, $account->getBalance());
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$account->addEntry(-700);
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$this->assertEquals(-200, $account->getBalance());
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}
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public function testExceedMaxLimit()
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{
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$account = new Account("123456", $maxCredit = 200);
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$this->setExpectedException("Exception");
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$account->addEntry(-1000);
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}
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}
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To enforce our rule we can now implement the assertion in `Account::addEntry`:
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[php]
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class Account
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{
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private function assertAcceptEntryAllowed($amount)
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{
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$futureBalance = $this->getBalance() + $amount;
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$allowedMinimalBalance = ($this->maxCredit * -1);
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if ($futureBalance < $allowedMinimalBalance) {
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throw new Exception("Credit Limit exceeded, entry is not allowed!");
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}
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}
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}
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We haven't talked to the entity manager for persistence of our account example before.
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You can call `EntityManager::persist($account)` and then `EntityManager::flush()`
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at any point to save the account to the database. All the nested `Entry` objects
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are automatically flushed to the database also.
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[php]
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$account = new Account("123456", 200);
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$account->addEntry(500);
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$account->addEntry(-200);
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$em->persist($account);
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$em->flush();
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The current implementation has a considerable drawback. To get the balance, we
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have to initialize the complete `Account::$entries` collection, possibly a very
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large one. This can considerably hurt the performance of your application.
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## Using an Aggregate Field
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To overcome the previously mentioned issue (initializing the whole entries collection)
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we want to add an aggregate field called "balance" on the Account and adjust the
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code in `Account::getBalance()` and `Account:addEntry()`:
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[php]
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class Account
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{
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/**
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* @Column(type="integer")
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*/
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private $balance = 0;
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public function getBalance()
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{
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return $this->balance;
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}
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public function addEntry($amount)
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{
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$this->assertAcceptEntryAllowed($amount);
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$e = new Entry($this, $amount);
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$this->entries[] = $e;
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$this->balance += $amount;
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return $e;
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}
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}
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This is a very simple change, but all the tests still pass. Our account entities return
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the correct balance. Now calling the `Account::getBalance()` method will not occur the
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overhead of loading all entries anymore. Adding a new Entry to the `Account::$entities`
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will also not initialize the collection internally.
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Adding a new entry is therefore very performant and explicitly hooked into the domain model.
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It will only update the account with the current balance and insert the new entry into the database.
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## Tackling Race Conditions with Aggregate Fields
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Whenever you denormalize your database schema race-conditions can potentially lead to
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inconsistent state. See this example:
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[php]
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// The Account $accId has a balance of 0 and a max credit limit of 200:
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// request 1 account
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$account1 = $em->find('Bank\Entities\Account', $accId);
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// request 2 account
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$account2 = $em->find('Bank\Entities\Account', $accId);
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$account1->addEntry(-200);
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$account2->addEntry(-200);
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// now request 1 and 2 both flush the changes.
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The aggregate field `Account::$balance` is now -200, however the SUM over all
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entries amounts yields -400. A violation of our max credit rule.
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You can use both optimistic or pessimistic locking to save-guard
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your aggregate fields against this kind of race-conditions. Reading Eric Evans
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DDD carefully he mentions that the "Aggregate Root" (Account in our example)
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needs a locking mechanism.
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Optimistic locking is as easy as adding a version column:
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[php]
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class Amount
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{
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/** @Column(type="integer") @Version */
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private $version;
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}
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The previous example would then throw an exception in the face of whatever request
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saves the entity last (and would create the inconsistent state).
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Pessimistic locking requires an additional flag set on the `EntityManager::find()`
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call, enabling write locking directly in the database using a FOR UPDATE.
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[php]
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use Doctrine\DBAL\LockMode;
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$account = $em->find('Bank\Entities\Account', $accId, LockMode::PESSIMISTIC_READ);
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## Keeping Updates and Deletes in Sync
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The example shown in this article does not allow changes to the value in `Entry`,
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which considerably simplifies the effort to keep `Account::$balance` in sync.
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If your use-case allows fields to be updated or related entities to be removed
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you have to encapsulate this logic in your "Aggregate Root" entity and adjust
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the aggregate field accordingly.
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## Conclusion
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This article described how to obtain aggregate values using DQL or your domain model.
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It showed how you can easily add an aggregate field that offers serious performance
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benefits over iterating all the related objects that make up an aggregate value.
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Finally I showed how you can ensure that your aggregate fields do not get out
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of sync due to race-conditions and concurrent access. |