实体属性值模式（EAV 模式）

实体属性值（Entity--attribute--value EAV）模式，可以方便 PHP 实现 EAV 模型。

## 目的

实体属性值模型（Entity-attribute-value EAV）是一种用数据模型描述实体的属性（属性，参数），可以用来形容他们潜在巨大，但实际上将适用于给定的实体的数量是相对较少。 在数学中，这种模式被称为一个**稀疏矩阵** 。 EAV 也被称为对象的属性值的模式，垂直的数据库模型和开放式架构。

## UML

![实体属性值模式](/media/202203/2022-03-29_2225500.12088408366652015.png)

## 示例代码

### PHP

```php
class Entity
{
    /**
     * @var \SplObjectStorage
     */
    private $values;

/**
     * @var string
     */
    private $name;

/**
     * @param string $name
     * @param Value[] $values
     */
    public function __construct(string $name, $values)
    {
        $this->values = new SplObjectStorage();
        $this->name = $name;

foreach ($values as $value) {
            $this->values->attach($value);
        }
    }

public function __toString(): string
    {
        $text = [$this->name];

foreach ($this->values as $value) {
            $text[] = (string) $value;
        }

return join(', ', $text);
    }
}

class Attribute
{
    /**
     * @var SplObjectStorage
     */
    private $values;

/**
     * @var string
     */
    private $name;

public function __construct(string $name)
    {
        $this->values = new SplObjectStorage();
        $this->name = $name;
    }

public function addValue(Value $value)
    {
        $this->values->attach($value);
    }

/**
     * @return SplObjectStorage
     */
    public function getValues(): SplObjectStorage
    {
        return $this->values;
    }

public function __toString(): string
    {
        return $this->name;
    }
}

class Value
{
    /**
     * @var Attribute
     */
    private $attribute;

/**
     * @var string
     */
    private $name;

public function __construct(Attribute $attribute, string $name)
    {
        $this->name = $name;
        $this->attribute = $attribute;

$attribute->addValue($this);
    }

public function __toString(): string
    {
        return sprintf('%s: %s', $this->attribute, $this->name);
    }
}

class EAVTest extends TestCase
{
    public function testCanAddAttributeToEntity()
    {
        $colorAttribute = new Attribute('color');
        $colorSilver = new Value($colorAttribute, 'silver');
        $colorBlack = new Value($colorAttribute, 'black');

$memoryAttribute = new Attribute('memory');
        $memory8Gb = new Value($memoryAttribute, '8GB');

$entity = new Entity('MacBook Pro', [$colorSilver, $colorBlack, $memory8Gb]);

$this->assertEquals('MacBook Pro, color: silver, color: black, memory: 8GB', (string) $entity);
    }
}
```

## 参考

- [百度百科：实体属性值模型](https://baike.baidu.com/item/%E5%AE%9E%E4%BD%93%E5%B1%9E%E6%80%A7%E5%80%BC%E6%A8%A1%E5%9E%8B/9672037)
- [SQL 反模式：实体-属性-值](https://blog.csdn.net/dh858115/article/details/104203110)