我的Laravel模型上有一个雄辩的关系，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关系时，我能够很好地加载这种关系，但是当我急切地加载这种关系时它不起作用。
具体来说，我有Product模型。该产品可能是也可能不是其他产品的父母。如果产品的parent_id设置为0，那么该产品将被视为父产品（无论是否有子产品）。如果将parent_id设置为其他产品的ID，则该产品是子产品。我需要能够访问Product::with('parent')并知道parent关系将返回 本身（是，重复数据）或不同的产品（如果是孩子）。
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总是未定义的，因此即使它实际上是父产品，这种关系也只会自行返回。
在关系被急切加载之前，有没有办法访问模型的属性？在我返回关系之前，我考虑过在单独的查询中工作，但我意识到我无法访问产品的ID甚至运行该查询。
如果不可能，还有哪些其他方法可以解决这类问题？这似乎不能通过传统的多态关系来解决。我只有两个可能的想法：
belongsTo关系中添加某种约束。老实说，我不知道如何实现其中任何一个。我是以正确的方式来做这件事的吗？我有什么东西可以忽略吗？
在考虑了这个之后，我认为提出问题的最简单方法是：有没有办法在运行时为关系本身内的关系动态选择外键？当我调用关系时，我的用例不允许我使用急切的加载约束 - 约束需要应用于关系本身。
答案 0 :(得分：5)
由于渴望加载的方式起作用，因此没有任何东西可以用来运行SQL来完成你正在寻找的东西。
执行Product::with('parent')->get()时，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渴望加载的父项：
select * from `products` where `products`.`id` in (?, ?, ?)
参数数量（?）对应于第一个查询的结果数。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。这将处理急切的加载方面。此外，您还需要覆盖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新方法，该方法将使用您的新CustomBelongsTo关系，并更新您的parent()关系方法以使用此新方法，而不是基本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。