Imported from snow-gift111/orangehrm-ai-sdlc-capstone (.agents/skills/entities/SKILL.md). Install upstream with npx skills add snow-gift111/orangehrm-ai-sdlc-capstone --skill entities. Copyright stays with the author.

Defining entities

All entities live in the flat OrangeHRM\Entity\ namespace regardless of which plugin owns them. Each plugin contributes its own entity/ directory to the multi-path namespace — see the doctrine-bootstrap skill for the registration mechanics.

This skill covers everything from "annotate the class" to "wire up a Decorator." For querying these entities, see the daos skill. For DDL (creating the underlying table), see the migrations skill.

The class shell

<?php
namespace OrangeHRM\Entity;

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

/**
 * @ORM\Table(name="ohrm_widget")
 * @ORM\Entity
 */
class Widget
{
    // …
}

Five rules followed everywhere:

1. Namespace is OrangeHRM\Entity, never a plugin-specific sub-namespace. The flat namespace is a hard constraint — the framework discovers entities across all plugin entity/ directories via PSR-4.
2. Class-level annotations are @ORM\Entity and @ORM\Table(name="…"). @ORM\Entity is required; without it Doctrine ignores the class.
3. Docblock annotations only — no PHP 8 attributes. The bootstrap uses createAnnotationMetadataConfiguration (see doctrine-bootstrap skill). Mixing attributes silently breaks discovery.
4. use Doctrine\ORM\Mapping as ORM; at the top of every entity file.
5. No business logic. Entities are plain getters/setters + ORM mapping. Convenience methods that need DB access go on a sibling Decorator class (see below).

Table naming convention

• New tables: ohrm_<resource> (e.g. ohrm_widget, ohrm_data_group, ohrm_user_role, ohrm_api_permission).
• Legacy tables: hs_hr_<resource> (e.g. hs_hr_employee, hs_hr_emp_basicsalary, hs_hr_config). These were imported from the 4.x codebase via the V3_3_3 baseline migration. Don't rename them — too many SQL queries, FKs, and external integrations depend on the exact names.

When adding a new table for a new feature, use the ohrm_ prefix. The hs_hr_ prefix is historical only.

Columns

/**
 * @var string|null
 *
 * @ORM\Column(name="job_description", type="string", length=400, nullable=true)
 */
private ?string $jobDescription = null;

Conventions seen everywhere:

• @var docblock matches the typed property declaration. Always include both.
• name="…" — the DB column name. Often snake_case while the PHP property is camelCase.
• type="…" — Doctrine type constant. Common ones: integer, string, text, boolean, smallint, bigint, decimal, date, datetime, datetimetz, time, json, blob.
• length=… — required for string; for performance and to mirror the DB column. Common defaults: 100 for names, 255 for free text fields, 512 for ciphertext storage.
• nullable=true when the column allows null. Always pair with ?type and = null default in the property declaration.
• options={"default" : …} — column default at the DB level. Use sparingly; prefer PHP-side defaults in the constructor when possible.

Reserved-word column names

Backtick-quote them in the annotation:

/**
 * @ORM\Column(name="`key`", type="string", length=100)
 */
private string $key;

key, order, value, status — MySQL reserves these. The V5_0_0_beta migration renamed hs_hr_config.key to name precisely to avoid the issue ongoing.

JSON columns

Doctrine has a built-in json type that handles serialization/deserialization:

/**
 * @ORM\Column(name="additional_params", type="text", nullable=true,
 *             options={"comment" = "(DC2Type:json)"})
 */
private ?array $additionalParams = null;

The (DC2Type:json) marker in the comment is Doctrine's convention — it's how the schema tool round-trips the type when introspecting an existing column declared as TEXT. New JSON columns should use type="json" directly; the comment marker only appears on legacy columns that started as TEXT.

Legacy ENUM columns

Columns declared ENUM(...) in MySQL load as plain strings. The bootstrap registers enum → string at the platform level (see doctrine-bootstrap). Don't try to declare type="enum" — Doctrine has no such type. Pattern:

public const STATE_ACTIVE     = 'ACTIVE';
public const STATE_TERMINATED = 'TERMINATED';

/**
 * @ORM\Column(name="state", type="string", length=20)
 */
private string $state = self::STATE_ACTIVE;

Class constants document the allowed values; validators (Rules::IN) gate input; the column itself is just a string at the ORM level.

IDs

The codebase has one ID pattern, used by every entity:

/**
 * @var int
 *
 * @ORM\Column(name="id", type="integer", length=4)
 * @ORM\Id
 * @ORM\GeneratedValue(strategy="AUTO")
 */
private int $id;

strategy="AUTO" on MySQL becomes AUTO_INCREMENT. Don't use IDENTITY, SEQUENCE, UUID, or composite keys without a strong reason — the rest of the codebase assumes integer surrogate keys.

The property name is not always id — it follows the domain. Employee uses $empNumber (column emp_number), but UserRole uses $id. Whatever the property is, getters follow it (getEmpNumber(), getId()). Other entities reference the ID via that property name, e.g. @JoinColumn(name="employee_id", referencedColumnName="emp_number").

Unique constraints and indexes

Declared at the table level:

/**
 * @ORM\Table(
 *     name="ohrm_api_permission",
 *     uniqueConstraints={
 *         @ORM\UniqueConstraint(name="api_name", columns={"api_name"})
 *     }
 * )
 * @ORM\Entity
 */
class ApiPermission { … }

Multi-column unique constraints follow the same shape — list all columns in the columns={…} array. @ORM\Index works analogously for non-unique indexes (rare in this codebase; most indexes are added via migrations directly).

Constructor — initialize collections

Every collection-valued relation needs an ArrayCollection initialized in the constructor:

public function __construct()
{
    $this->dependents          = new ArrayCollection();
    $this->emergencyContacts   = new ArrayCollection();
    $this->skills              = new ArrayCollection();
    // … one per OneToMany / ManyToMany on the class
}

If you skip this, the relation is null until lazy-loaded — and code that adds to it before fetching from the DB will throw Call to a member function add() on null. The Employee entity initializes ~17 collections in its constructor; mirror that for any entity with many relations.

Relations

The four flavors all appear in the codebase. Reference Employee (Pim plugin) for the canonical examples.

ManyToOne — the most common

The "many" side owns the foreign key:

/**
 * @var JobTitle|null
 *
 * @ORM\ManyToOne(targetEntity="OrangeHRM\Entity\JobTitle", inversedBy="employees")
 * @ORM\JoinColumn(name="job_title_code", referencedColumnName="id", nullable=true)
 */
private ?JobTitle $jobTitle = null;

• targetEntity is the related entity's FQCN.
• inversedBy="employees" is optional but conventional — points at the OneToMany collection on the other side. Doctrine uses this to keep both sides in sync when you set one.
• @JoinColumn names the FK column. referencedColumnName defaults to id if omitted — be explicit when the target's ID column is non-standard (e.g. emp_number).
• nullable=true when the relation is optional. Always pair with ?Type and = null default.

OneToMany — the inverse of ManyToOne

The collection side. Always paired with a ManyToOne on the other entity.

/**
 * @var Employee[]
 *
 * @ORM\OneToMany(targetEntity="OrangeHRM\Entity\Employee", mappedBy="jobTitle")
 */
private iterable $employees;

• mappedBy="jobTitle" — names the property on the other entity that owns the FK. Required.
• Property type is iterable in annotations + initialized to ArrayCollection in the constructor.
• @var docblock uses array notation (Employee[]) for static analysis hints.

ManyToMany — needs a JoinTable

Owning side declares the join table:

/**
 * @var Location[]
 *
 * @ORM\ManyToMany(targetEntity="OrangeHRM\Entity\Location", inversedBy="employees")
 * @ORM\JoinTable(
 *     name="hs_hr_emp_locations",
 *     joinColumns={@ORM\JoinColumn(name="emp_number", referencedColumnName="emp_number")},
 *     inverseJoinColumns={@ORM\JoinColumn(name="location_id", referencedColumnName="id")}
 * )
 */
private iterable $locations;

Inverse side just uses mappedBy:

/**
 * @ORM\ManyToMany(targetEntity="OrangeHRM\Entity\Employee", mappedBy="locations")
 */
private iterable $employees;

OneToOne

/**
 * @var EmpPicture|null
 *
 * @ORM\OneToOne(targetEntity="OrangeHRM\Entity\EmpPicture", mappedBy="employee")
 */
private ?EmpPicture $empPicture = null;

Or on the owning side (with @JoinColumn):

/**
 * @ORM\OneToOne(targetEntity="OrangeHRM\Entity\JobSpecificationAttachment", mappedBy="jobTitle",
 *               cascade={"persist", "remove"})
 */
private ?JobSpecificationAttachment $jobSpecificationAttachment = null;

Self-referential M:N (Employee.supervisors)

Same class on both sides. The owning side declares the join table; the inverse uses mappedBy:

// Owning side
@ORM\ManyToMany(targetEntity="OrangeHRM\Entity\Employee", inversedBy="subordinates")
@ORM\JoinTable(name="hs_hr_emp_reportto", joinColumns=…, inverseJoinColumns=…)
private iterable $supervisors;

// Inverse side
@ORM\ManyToMany(targetEntity="OrangeHRM\Entity\Employee", mappedBy="supervisors")
private iterable $subordinates;

cascade and orphanRemoval

Used sparingly. cascade={"persist", "remove"} is the most common pair, mostly on OneToOne where the child is logically owned by the parent (JobSpecificationAttachment → JobTitle). Most relations have no cascade — services explicitly manage related-entity lifecycle.

orphanRemoval=true exists but is rare in this codebase. Don't add cascades reflexively; only when the related entity has no meaning outside the parent.

Entity lifecycle — @ORM\EntityListeners, not lifecycle callbacks

The codebase does NOT use @HasLifecycleCallbacks / @PrePersist / @PreUpdate on the entity class itself. Zero usages. The lifecycle pattern is always external listener classes registered via @ORM\EntityListeners.

/**
 * @ORM\Table(name="hs_hr_employee")
 * @ORM\Entity
 * @ORM\EntityListeners({"OrangeHRM\Entity\Listener\EmployeeListener"})
 */
class Employee { … }

The listener class:

namespace OrangeHRM\Entity\Listener;

use Doctrine\ORM\Event\PreUpdateEventArgs;
use Doctrine\ORM\Event\LifecycleEventArgs;
use OrangeHRM\Entity\Employee;

class EmployeeListener extends BaseListener
{
    public function preUpdate(Employee $employee, PreUpdateEventArgs $eventArgs): void
    {
        if ($this->encryptionEnabled() && $eventArgs->hasChangedField('ssnNumber')) {
            $employee->setSsnNumber(
                $this->getCryptographer()->encrypt($employee->getSsnNumber())
            );
        }
    }

    public function postUpdate(Employee $employee, LifecycleEventArgs $eventArgs): void
    {
        if ($this->encryptionEnabled()) {
            $employee->setSsnNumber(
                $this->getCryptographer()->decrypt($employee->getSsnNumber())
            );
        }
    }
}

Three real listeners exist: EmployeeListener (SSN encryption), EmployeeSalaryListener (basic-salary encryption), EmailConfigurationListener (SMTP password encryption).

BaseListener (src/plugins/orangehrmCorePlugin/entity/Listener/BaseListener.php) is a thin abstract:

abstract class BaseListener
{
    use EncryptionHelperTrait;  // gives $this->encryptionEnabled(), $this->getCryptographer()
}

Why external listeners and not @HasLifecycleCallbacks? Because the lifecycle logic needs a service (Cryptographer) that itself depends on config. Listeners can pull services in via traits; lifecycle callbacks on the entity itself can't easily, and they couple business logic to the data shape.

The pattern: encrypt on preUpdate (and prePersist if needed), decrypt on postUpdate (and postLoad) so the in-memory entity stays readable. Symmetric round-trip.

The Decorator pattern — entity-adjacent logic with DB access

Entities are plain POPOs. Logic that involves the EntityManager — looking up a related entity by ID, formatting a date with the user's timezone, computing the employee's STATE from termination + active flags — lives on a sibling Decorator class.

The trait

OrangeHRM\Entity\Decorator\DecoratorTrait is used on the entity:

namespace OrangeHRM\Entity;

use OrangeHRM\Entity\Decorator\DecoratorTrait;
use OrangeHRM\Entity\Decorator\EmployeeDecorator;

/**
 * @method EmployeeDecorator getDecorator()    ← IDE hint for the typed return
 *
 * @ORM\Table(name="hs_hr_employee")
 * @ORM\Entity
 */
class Employee
{
    use DecoratorTrait;
    // …
}

The trait gives you a single method:

public function getDecorator(): object
{
    if (is_null($this->entityDecorator)) {
        $decoratorClassName = $this->getDecoratorClassName();
        $this->entityDecorator = new $decoratorClassName($this);
    }
    return $this->entityDecorator;
}

It resolves the Decorator class name by reflection: OrangeHRM\Entity\Decorator\<EntityName>Decorator. Override getDecoratorClassName() if you need something different (rarely needed).

The @method PHPDoc on the entity tells IDEs the precise return type — without it, IDEs see object and lose autocomplete.

The Decorator class

namespace OrangeHRM\Entity\Decorator;

use OrangeHRM\Core\Traits\ORM\EntityManagerHelperTrait;
use OrangeHRM\Core\Traits\Service\DateTimeHelperTrait;
use OrangeHRM\Entity\Employee;
use OrangeHRM\Entity\JobTitle;

class EmployeeDecorator
{
    use EntityManagerHelperTrait;
    use DateTimeHelperTrait;

    public function __construct(private Employee $employee) {}

    public function setJobTitleById(?int $id): void
    {
        $jobTitle = is_null($id) ? null : $this->getReference(JobTitle::class, $id);
        $this->employee->setJobTitle($jobTitle);
    }

    public function getJoinedDate(): ?string {
        $date = $this->employee->getJoinedDate();
        return is_null($date) ? null : $this->getDateTimeHelper()->formatDate($date);
    }

    public function getState(): string { /* compute from termination + status */ }

    public function getFullName(): string { /* compose first + middle + last */ }
}

The Decorator is where you put:

• setXById($id) shortcuts — fetch the related entity via getReference() and call the real setter. Used heavily during API request body binding so callers don't have to fetch FK targets themselves.
• Formatters that need DI — date formatters (need user timezone), translators, etc.
• Computed properties — getState() (Active / Terminated / Not Exist), getFullName(), etc.

When to use a Decorator vs. a Service

• Decorator if the logic is intrinsic to the entity and naturally written as "operations on this object." Employee->getFullName(). The Decorator gives you the entity in $this->employee.
• Service if the logic involves multiple unrelated entities, complex workflow, or external integrations. EmployeeService::transferToLocation($empNumber, $locationId) belongs in a Service, not a Decorator.

Decorators are widely used — 11+ in PimPlugin alone. The pattern is well-established; mirror it for any new entity that has both DB-touching helpers and a need to stay POPO.

Tree-shaped entities — the NestedSet helper

For traditional MPTT (nested set) trees, src/lib/orm/NestedSet/ provides:

• NestedSetInterface, NodeInterface — contracts
• NestedSetTrait — lft/rgt/treeId accessors
• Node — a value object

One real consumer: Subunit (the org structure tree, orangehrmAdminPlugin/entity/Subunit.php).

class Subunit implements NestedSetInterface
{
    use NestedSetTrait;
    // … usual entity stuff, plus lft/rgt/depth columns
}

If you need a hierarchy with frequent "all descendants of X" queries, NestedSet is the existing pattern. For simple parent/child without deep traversal, a regular self-referential ManyToOne is fine and lower-friction.

What's NOT used (don't propose these)

• PHP 8 attributes for mapping. Doctrine supports them, but the bootstrap uses createAnnotationMetadataConfiguration so only docblock annotations are scanned. A partial migration silently breaks the migrated entities. (The doctrine-bootstrap skill anchors the exact Doctrine version to src/composer.json.)
• @HasLifecycleCallbacks / @PrePersist / @PreUpdate / etc. on the entity class itself. Use @ORM\EntityListeners instead — every existing example uses the external listener pattern.
• Custom repositoryClass on @Entity. The DAO layer (see daos skill) replaces Doctrine's repository-per-entity feature; entities don't reference a repository class.
• Embedded objects (@Embedded). Not used.
• Inheritance mapping (@InheritanceType, @DiscriminatorColumn). Not used. The few class hierarchies in the codebase (e.g. report definitions) use separate tables and separate entities, not single-table or joined-table inheritance.
• UUID IDs. Every entity uses integer AUTO_INCREMENT.

Recipes

Recipe 1 — A simple entity with one M2O relation

<?php
namespace OrangeHRM\Entity;

use Doctrine\ORM\Mapping as ORM;
use OrangeHRM\Entity\Decorator\DecoratorTrait;
use OrangeHRM\Entity\Decorator\WidgetDecorator;

/**
 * @method WidgetDecorator getDecorator()
 *
 * @ORM\Table(name="ohrm_widget")
 * @ORM\Entity
 */
class Widget
{
    use DecoratorTrait;

    /**
     * @var int
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private int $id;

    /**
     * @var string
     * @ORM\Column(name="name", type="string", length=100)
     */
    private string $name;

    /**
     * @var Employee|null
     * @ORM\ManyToOne(targetEntity="OrangeHRM\Entity\Employee")
     * @ORM\JoinColumn(name="owner_emp_number", referencedColumnName="emp_number", nullable=true)
     */
    private ?Employee $owner = null;

    // getId, getName/setName, getOwner/setOwner …
}

Migration creates the table (see migrations skill). DAO queries it (see daos skill).

Recipe 2 — Entity with one-to-many + initialized collection

/**
 * @ORM\Table(name="ohrm_widget_category")
 * @ORM\Entity
 */
class WidgetCategory
{
    // … id, name …

    /**
     * @var Widget[]
     * @ORM\OneToMany(targetEntity="OrangeHRM\Entity\Widget", mappedBy="category")
     */
    private iterable $widgets;

    public function __construct()
    {
        $this->widgets = new ArrayCollection();
    }

    public function getWidgets(): iterable { return $this->widgets; }
}

The matching Widget adds:

/**
 * @ORM\ManyToOne(targetEntity="OrangeHRM\Entity\WidgetCategory", inversedBy="widgets")
 * @ORM\JoinColumn(name="category_id", referencedColumnName="id")
 */
private ?WidgetCategory $category = null;

Recipe 3 — Decorator class with setXById shortcuts

namespace OrangeHRM\Entity\Decorator;

use OrangeHRM\Core\Traits\ORM\EntityManagerHelperTrait;
use OrangeHRM\Entity\Employee;
use OrangeHRM\Entity\Widget;
use OrangeHRM\Entity\WidgetCategory;

class WidgetDecorator
{
    use EntityManagerHelperTrait;

    public function __construct(private Widget $widget) {}

    public function setOwnerByEmpNumber(?int $empNumber): void
    {
        $employee = is_null($empNumber) ? null : $this->getReference(Employee::class, $empNumber);
        $this->widget->setOwner($employee);
    }

    public function setCategoryById(?int $id): void
    {
        $category = is_null($id) ? null : $this->getReference(WidgetCategory::class, $id);
        $this->widget->setCategory($category);
    }
}

API handler then uses these shortcuts to bind from request body without fetching FK targets first:

$widget->getDecorator()->setCategoryById($categoryId);
$widget->getDecorator()->setOwnerByEmpNumber($ownerEmpNumber);

getReference() (from EntityManagerHelperTrait) returns a proxy — no SELECT until the related entity's data is accessed. Cheap for the FK-binding case.

Recipe 4 — EntityListener for encrypting a sensitive field

// Entity
/**
 * @ORM\EntityListeners({"OrangeHRM\Entity\Listener\OAuthClientListener"})
 */
class OAuthClient { /* clientSecret column … */ }

// Listener
namespace OrangeHRM\Entity\Listener;

use Doctrine\ORM\Event\LifecycleEventArgs;
use Doctrine\ORM\Event\PreUpdateEventArgs;
use OrangeHRM\Entity\OAuthClient;

class OAuthClientListener extends BaseListener
{
    public function prePersist(OAuthClient $client, LifecycleEventArgs $args): void
    {
        if ($this->encryptionEnabled()) {
            $client->setClientSecret($this->getCryptographer()->encrypt($client->getClientSecret()));
        }
    }

    public function preUpdate(OAuthClient $client, PreUpdateEventArgs $args): void
    {
        if ($this->encryptionEnabled() && $args->hasChangedField('clientSecret')) {
            $client->setClientSecret($this->getCryptographer()->encrypt($client->getClientSecret()));
        }
    }

    public function postLoad(OAuthClient $client, LifecycleEventArgs $args): void
    {
        if ($this->encryptionEnabled()) {
            $client->setClientSecret($this->getCryptographer()->decrypt($client->getClientSecret()));
        }
    }

    public function postUpdate(OAuthClient $client, LifecycleEventArgs $args): void
    {
        if ($this->encryptionEnabled()) {
            $client->setClientSecret($this->getCryptographer()->decrypt($client->getClientSecret()));
        }
    }
}

The column type stays string with widened length (≥512) to accommodate ciphertext. The migration that introduces the field should size it correctly from the start.

Checklists

Add a new entity

• File at src/plugins/orangehrm{X}Plugin/entity/<Name>.php in namespace OrangeHRM\Entity;
• @ORM\Entity + @ORM\Table(name="ohrm_<resource>") on class
• Plugin's entity dir is in OrangeHRM\Entity\ PSR-4 in src/composer.json (see doctrine-bootstrap skill)
• composer dump-autoload -d src after touching composer.json
• Integer AUTO_INCREMENT ID — @ORM\Id + @ORM\GeneratedValue(strategy="AUTO")
• All collection-valued relations initialized to new ArrayCollection() in __construct
• Getters/setters for every persisted property + each relation
• If logic needs DB access: sibling Decorator class in Entity\Decorator\ + use DecoratorTrait + @method PHPDoc hint
• If lifecycle hooks needed: @ORM\EntityListeners({"…"}) + listener class extending BaseListener
• Migration to create the table (see migrations skill)

Add a relation between entities

• Decide owning side — the one with the FK column. M2O ↔ O2M: owning is the M2O side. M2M: arbitrary, but JoinTable lives there.
• Owning side: @ManyToOne / @OneToOne / @ManyToMany + @JoinColumn (or @JoinTable for M2M) + optional inversedBy="…"
• Inverse side: @OneToMany / @OneToOne / @ManyToMany + mappedBy="…"
• Inverse side collection: initialize in constructor
• nullable=true on @JoinColumn paired with ?Type and = null if relation is optional
• Add a Decorator method setXById($id) if FK will be set from a body param
• Schema migration (see migrations skill)

Add an EntityListener

• Class in src/plugins/orangehrm{X}Plugin/entity/Listener/ extending BaseListener
• Listener methods take ($entity, $eventArgs) — type-hint the entity for IDE help
• Use PreUpdateEventArgs::hasChangedField('property') to short-circuit when the relevant field didn't change
• For encryption: symmetric pair — encrypt in prePersist / preUpdate, decrypt in postLoad / postUpdate so the in-memory entity stays readable
• Register the listener on the entity: @ORM\EntityListeners({"OrangeHRM\Entity\Listener\<Name>Listener"})

Things that bite

• Forgetting to initialize a collection in __construct — first $entity->getThings()->add(...) on a fresh entity throws "Call to a member function add() on null." Doctrine populates collections during hydration but constructors run before hydration.
• Using a sub-namespace for entities (e.g. OrangeHRM\Pim\Entity\Foo). Doctrine's path scanner finds it, but the framework's PSR-4 + multi-path lookup expects flat OrangeHRM\Entity\. Anywhere else and references break.
• Adding @HasLifecycleCallbacks + @PrePersist to an entity. The codebase uses @EntityListeners exclusively; mixing patterns produces inconsistent maintenance debt and other devs won't look for callbacks on the entity itself.
• mappedBy vs inversedBy confusion — owning side has inversedBy, inverse side has mappedBy. Getting it backwards still "works" for reads but breaks Doctrine's change-tracking when setting one side and expecting the other to sync.
• @JoinColumn referencing the wrong column when target's ID column is non-standard — referencedColumnName defaults to id; Employee's ID column is emp_number. Always specify explicitly when joining to Employee.
• Relying on getDecorator() without the @method PHPDoc hint — works at runtime but IDEs see object and lose autocomplete. Always add the @method <Name>Decorator getDecorator() line.
• Putting business logic on the entity directly instead of in a Decorator or Service. Future you (or PHPCS-fix) will hate it; entities should stay POPO for clean serialization.
• Setting two M2M sides explicitly — when you call $a->addB($b) from the owning side, Doctrine syncs the inverse via inversedBy. Calling $b->getAs()->add($a) separately can produce duplicate join rows.