同一模型上的 Laravel 父子关系

Calling the relationship function (->children()) will return an instance of the relation class. You either need to call then get() or just use the property:

$children = $category->children()->get();
// or
$children = $category->children;

Further explanation

实际上，children()和children是截然不同的东西.children()只调用您为您的关系定义的方法.该方法返回HasMany的对象.您可以使用它来应用进一步的查询方法.例如:

$category->children()->orderBy('firstname')->get();

现在访问property children的工作方式不同了.你从来没有定义过，所以拉威尔在背景中做了一些魔术.

Let's have a look at Illuminate\Database\Eloquent\Model:

public function __get($key)
{
    return $this->getAttribute($key);
}

The __get function is called when you try to access a property on a PHP object that doesn't actually exist.

public function getAttribute($key)
{
    $inAttributes = array_key_exists($key, $this->attributes);

    // If the key references an attribute, we can just go ahead and return the
    // plain attribute value from the model. This allows every attribute to
    // be dynamically accessed through the _get method without accessors.
    if ($inAttributes || $this->hasGetMutator($key))
    {
        return $this->getAttributeValue($key);
    }

    // If the key already exists in the relationships array, it just means the
    // relationship has already been loaded, so we'll just return it out of
    // here because there is no need to query within the relations twice.
    if (array_key_exists($key, $this->relations))
    {
        return $this->relations[$key];
    }

    // If the "attribute" exists as a method on the model, we will just assume
    // it is a relationship and will load and return results from the query
    // and hydrate the relationship's value on the "relationships" array.
    $camelKey = camel_case($key);

    if (method_exists($this, $camelKey))
    {
        return $this->getRelationshipFromMethod($key, $camelKey);
    }
}

然后在getAttribute first中有一些代码判断"normal"属性，然后返回.最后，在方法的末尾，如果有定义的关系，则调用getRelationshipFromMethod方法.

It will then retrieve the result of the relationship and return that.