我在我的 Laravel 模型上有一个 Eloquent 关系，它是动态的——也就是说，特定数据库字段的值决定了哪个模型将被加载。当我第一次实例化模型实例然后引用该关系时，我能够很好地加载该关系，但是当我急切地加载该关系时它不起作用。

具体来说，我有一个 Product 模型。该产品可能是也可能不是另一个产品的父产品。如果产品的 parent_id 设置为 0，则该产品被视为父部件(无论它是否有子部件)。如果 parent_id 设置为不同产品的 ID，则该产品是子产品。我需要能够访问 Product::with('parent') 并且知道 parent 关系将返回 either 本身(是的，重复数据)或不同的产品(如果是子产品)。

到目前为止，这是我的关系:

public function parent()
{
    if ($this->parent_id > 0) {
        return $this->belongsTo('App\Product', 'parent_id', 'id');
    } else {
        return $this->belongsTo('App\Product', 'id', 'id');
    }
}

当我急切加载时，$this->parent_id 始终未定义，因此即使它实际上是父产品，该关系也只会返回自身。

有什么方法可以在关系被预加载之前访问模型的属性吗？我考虑过在返回关系之前使用单独的查询，但我意识到我无权访问产品的 ID 甚至无法运行该查询。

如果那不可能，还有哪些其他方法可以解决此类问题？这似乎无法通过传统的多态关系来解决。我只有两个可能的想法:

• 向动态确定外键的 belongsTo 关系添加某种约束。
• 创建我自己的自定义关系，该关系使用基于不同数据库字段的外键。

老实说，我不知道我将如何实现其中任何一个。我这样做是对的吗？我忽略了什么吗？

在仔细考虑之后，我认为提出问题的最简单方式是:是否有任何方法可以在运行时为关系本身内部的关系动态选择外键？我的用例不允许我在调用关系时使用预加载约束 - 约束需要应用于关系本身。

最佳答案

由于预先加载的工作方式，您无法真正对正在运行的 SQL 执行任何操作来完成您要查找的内容。

当您执行 Product::with('parent')->get() 时，它会运行两个查询。

首先，它运行查询以获取所有产品:

select * from `products`

接下来，它运行一个查询来获取急切加载的父级:

select * from `products` where `products`.`id` in (?, ?, ?)

参数的数量 (?) 对应于第一个查询的结果数量。检索到第二组模型后，match() 函数用于将对象相互关联。

为了做你想做的事，你将不得不创建一个新的关系并覆盖 match() 方法。这将处理预加载方面。此外，您需要覆盖 addConstraints 方法来处理延迟加载方面。

首先，创建一个自定义关系类:

class CustomBelongsTo extends BelongsTo
{
    // Override the addConstraints method for the lazy loaded relationship.
    // If the foreign key of the model is 0, change the foreign key to the
    // model's own key, so it will load itself as the related model.

    /**
     * Set the base constraints on the relation query.
     *
     * @return void
     */
    public function addConstraints()
    {
        if (static::$constraints) {
            // For belongs to relationships, which are essentially the inverse of has one
            // or has many relationships, we need to actually query on the primary key
            // of the related models matching on the foreign key that's on a parent.
            $table = $this->related->getTable();

            $key = $this->parent->{$this->foreignKey} == 0 ? $this->otherKey : $this->foreignKey;

            $this->query->where($table.'.'.$this->otherKey, '=', $this->parent->{$key});
        }
    }

    // Override the match method for the eager loaded relationship.
    // Most of this is copied from the original method. The custom
    // logic is in the elseif.

    /**
     * Match the eagerly loaded results to their parents.
     *
     * @param  array   $models
     * @param  \Illuminate\Database\Eloquent\Collection  $results
     * @param  string  $relation
     * @return array
     */
    public function match(array $models, Collection $results, $relation)
    {
        $foreign = $this->foreignKey;

        $other = $this->otherKey;

        // First we will get to build a dictionary of the child models by their primary
        // key of the relationship, then we can easily match the children back onto
        // the parents using that dictionary and the primary key of the children.
        $dictionary = [];

        foreach ($results as $result) {
            $dictionary[$result->getAttribute($other)] = $result;
        }

        // Once we have the dictionary constructed, we can loop through all the parents
        // and match back onto their children using these keys of the dictionary and
        // the primary key of the children to map them onto the correct instances.
        foreach ($models as $model) {
            if (isset($dictionary[$model->$foreign])) {
                $model->setRelation($relation, $dictionary[$model->$foreign]);
            }
            // If the foreign key is 0, set the relation to a copy of the model
            elseif($model->$foreign == 0) {
                // Make a copy of the model.
                // You don't want recursion in your relationships.
                $copy = clone $model;

                // Empty out any existing relationships on the copy to avoid
                // any accidental recursion there.
                $copy->setRelations([]);

                // Set the relation on the model to the copy of itself.
                $model->setRelation($relation, $copy);
            }
        }

        return $models;
    }
}

创建自定义关系类后，您需要更新模型以使用此自定义关系。在您的模型上创建一个将使用新的 CustomBelongsTo 关系的新方法，并更新您的 parent() 关系方法以使用此新方法，而不是基础 belongsTo() 方法。

class Product extends Model
{

    // Update the parent() relationship to use the custom belongsto relationship
    public function parent()
    {
        return $this->customBelongsTo('App\Product', 'parent_id', 'id');
    }

    // Add the method to create the CustomBelongsTo relationship. This is
    // basically a copy of the base belongsTo method, but it returns
    // a new CustomBelongsTo relationship instead of the original BelongsTo relationship
    public function customBelongsTo($related, $foreignKey = null, $otherKey = null, $relation = null)
    {
        // If no relation name was given, we will use this debug backtrace to extract
        // the calling method's name and use that as the relationship name as most
        // of the time this will be what we desire to use for the relationships.
        if (is_null($relation)) {
            list($current, $caller) = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 2);

            $relation = $caller['function'];
        }

        // If no foreign key was supplied, we can use a backtrace to guess the proper
        // foreign key name by using the name of the relationship function, which
        // when combined with an "_id" should conventionally match the columns.
        if (is_null($foreignKey)) {
            $foreignKey = Str::snake($relation).'_id';
        }

        $instance = new $related;

        // Once we have the foreign key names, we'll just create a new Eloquent query
        // for the related models and returns the relationship instance which will
        // actually be responsible for retrieving and hydrating every relations.
        $query = $instance->newQuery();

        $otherKey = $otherKey ?: $instance->getKeyName();

        return new CustomBelongsTo($query, $this, $foreignKey, $otherKey, $relation);
    }
}

公平警告，这些都没有经过测试。

关于php - Laravel 动态关系 - 在急切加载时访问模型属性，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/35438254/