laravel 详细的文档有吗，英文版的也可以

没有 放弃吧 兄弟 :smirk:

看源码不就知道了

/**
 * Define an inverse one-to-one or many relationship.
 *
 * @param  string  $related
 * @param  string|null  $foreignKey
 * @param  string|null  $ownerKey
 * @param  string|null  $relation
 * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
 */
public function belongsTo($related, $foreignKey = null, $ownerKey = 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)) {
        $relation = $this->guessBelongsToRelation();
    }

    $instance = $this->newRelatedInstance($related);

    // 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).'_'.$instance->getKeyName();
    }

    // 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.
    $ownerKey = $ownerKey ?: $instance->getKeyName();

    return $this->newBelongsTo(
        $instance->newQuery(), $this, $foreignKey, $ownerKey, $relation
    );
}

加查询条件可以：

return $this->belongsTo(Pricing::class, 'pricing_id', 'id')->where('', ''); //全局加上
Product::with(['pricings' => function($query) {
    $query->where('your quiry condition')
}])

1. 看源码
2. API 文档