Laravel父/子关系在同一型号上

Question

设置和虚拟数据

我有一个名为Category的简单模型,它具有以下模式:

|----------------------------------------------|
| cat_id   | cat_name    | parent_id           |
|----------------------------------------------|
|   1      | Home        |   0                 |
|----------------------------------------------|
|   2      | Products    |   1                 |
|----------------------------------------------| 
|   3      | Services    |   1                 |
|----------------------------------------------|
|   4      | Product A   |   2                 |
|----------------------------------------------|
|   5      | Product B   |   2                 |
|----------------------------------------------|

期望的输出

所以你可以看到我们将得到一个非常直接的层次结构,如下所示:

Home
  - Products
      - Product A
      - Product B
  - Services

问题

我正在尝试在Laravel 4.2中映射这种关系,以便我可以查询模型并获取其父项(它将始终具有父项)和子项类别(如果它们存在).

我使用以下方法在Category模型中定义了关系:

public function children()
{
    return $this->hasMany('Category', 'parent_id', 'cat_id');
}
public function parent()
{
    return $this->belongsTo('Category', 'parent_id');
}

问题

我可以使用,让父名称正常工作

$category = Category::findOrFail($id);
return $category->parent->cat_name;

但是,我不明白如何获取子对象.

我试过了:

$category = Category::findOrFail($id);
$children = $category->children();

但是当我dd($ children)它没有输出我所期望的.

Answer 1

调用关系函数(->children())将返回关系类的实例.您需要调用then get()或只使用该属性:

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

进一步说明

实际上children()并且children是非常不同的东西.children()只需调用您为关系定义的方法.该方法返回一个对象HasMany.您可以使用它来应用更多查询方法.例如:

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

现在访问该属性的 children方式不同.你从未定义它,所以Laravel在后台做了一些魔术.

我们来看看Illuminate\Database\Eloquent\Model:

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

__get当您尝试访问实际不存在的PHP对象上的属性时,将调用该函数.

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getAttribute首先是一些检查"正常"属性然后返回的代码.最后,在方法结束时,如果定义了关系方法getRelationshipFromMethod被调用.

然后它将检索关系的结果并返回该结果.

Answer 2

在模型中设置它并尝试：

public function children()
{
    return $this->hasMany(self::class, 'parent_id');
}

public function grandchildren()
{
    return $this->children()->with('grandchildren');
}