Вопрос: Как правильно связать несколько таблиц?

Лучший ответ:
<?php
$model = \App\Models\Building::where('id', 1)->with('street.settlement.subregion.region.country')->first();
echo $model->address;
<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Building extends Model
{
    public function street()
    {
        return $this->belongsTo('App\Models\Street');
    }

    public function getAddressAttribute()
    {
        $parts = [
            $this->street->settlement->subregion->region->country->title,
            $this->street->settlement->subregion->region->title,
            $this->street->settlement->subregion->title,
            $this->street->settlement->title,
            $this->street->title,
            $this->title,
        ];
        return join(', ', $parts);
    }
}

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Street extends Model
{
    public function settlement()
    {
        return $this->belongsTo('App\Models\Settlement');
    }
}

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Settlement extends Model
{
    public function subregion()
    {
        return $this->belongsTo('App\Models\Subregion');
    }
}

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Subregion extends Model
{
    public function region()
    {
        return $this->belongsTo('App\Models\Region');
    }
}

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Region extends Model
{
    public function country()
    {
        return $this->belongsTo('App\Models\Country');
    }
}

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Country extends Model
{

}

DROP TABLE IF EXISTS `buildings`;
CREATE TABLE `buildings` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `street_id` int DEFAULT NULL,
  `title` varchar(255) COLLATE utf8mb4_ru_0900_ai_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

DROP TABLE IF EXISTS `countries`;
CREATE TABLE `countries` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `title` varchar(255) COLLATE utf8mb4_ru_0900_ai_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

DROP TABLE IF EXISTS `regions`;
CREATE TABLE `regions` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `country_id` int DEFAULT NULL,
  `title` varchar(255) COLLATE utf8mb4_ru_0900_ai_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

DROP TABLE IF EXISTS `settlements`;
CREATE TABLE `settlements` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `subregion_id` int DEFAULT NULL,
  `title` varchar(255) COLLATE utf8mb4_ru_0900_ai_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

DROP TABLE IF EXISTS `streets`;
CREATE TABLE `streets` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `settlement_id` int DEFAULT NULL,
  `title` varchar(255) COLLATE utf8mb4_ru_0900_ai_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

DROP TABLE IF EXISTS `subregions`;
CREATE TABLE `subregions` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `region_id` int DEFAULT NULL,
  `title` varchar(255) COLLATE utf8mb4_ru_0900_ai_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_ru_0900_ai_ci;

INSERT INTO `buildings` (`id`, `street_id`, `title`) VALUES
('1', '1', '43');

INSERT INTO `countries` (`id`, `title`) VALUES
('1', 'Российская Федерация');

INSERT INTO `regions` (`id`, `country_id`, `title`) VALUES
('1', '1', 'Алтайский край');

INSERT INTO `settlements` (`id`, `subregion_id`, `title`) VALUES
('1', '1', 'г. Барнаул');

INSERT INTO `streets` (`id`, `settlement_id`, `title`) VALUES
('1', '1', 'ул. Пушкина');

INSERT INTO `subregions` (`id`, `region_id`, `title`) VALUES
('1', '1', 'Алтайский район');

Худший ответ:
Евгений, сорри за наглость.
Можно это прописать так, чтоб находясь в другой модели получил другие адреса например с Subregion получаю Subregion, Region и Country или с Region получаю Region и Country.

Имею ввиду не хардкод, а черзе trait