Laravel relationships with multiple linking tables.

Document.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Document extends Model
{
    protected $guarded = ['id'];
    public $timestamps = false;

    public function fileLocations()
    {
        return $this->hasMany(FileLocation::class, 'foreignId', 'documentId');
    }

}

File.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class File extends Model
{
    protected $guarded = ['id'];
    public $timestamps = false;

    public function fileUpload()
    {
        return $this->hasOne(FileUploadLocation::class, 'fileId');
    }
}

FileLocation.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class FileLocation extends Model
{
    protected $primaryKey = 'foreignId';
    protected $guarded = ['foreignId'];
    public $timestamps = false;

    public function document()
    {
        return $this->belongsTo(Document::class, 'foreignId', 'documentId');
    }

    public function fileUpload()
    {
        return $this->hasOne(FileUploadLocation::class, 'locationId', 'id');
    }

}

FileUploadLocation.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class FileUploadLocation extends Model
{
    protected $primaryKey = 'locationId';
    protected $guarded = ['locationId'];
    public $timestamps = false;

    public function fileLocation()
    {
        return $this->hasOne(FileLocation::class, 'id', 'locationId');
    }

    public function file()
    {
        return $this->hasOne(File::class, 'id', 'fileId');
    }
}

migrations_2017_12_08_223949_create_documents_table.php

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateDocumentsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('documents', function (Blueprint $table) {
            $table->increments('id');
            $table->unsignedInteger('documentId')->nullable();
            $table->string('name');

            $table->foreign('documentId')
                ->references('foreignId')
                ->on('file_locations');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('documents');
    }
}

migrations_2017_12_08_224007_create_file_locations_table.php

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateFileLocationsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('file_locations', function (Blueprint $table) {
            $table->increments('foreignId');
            $table->unsignedInteger('id')->nullable();

            $table->foreign('id')
                ->references('locationId')
                ->on('file_upload_locations');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('file_locations');
    }
}

migrations_2017_12_08_224012_create_file_upload_locations_table.php

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateFileUploadLocationsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('file_upload_locations', function (Blueprint $table) {
            $table->increments('locationId');
            $table->unsignedInteger('fileId');

            $table->foreign('fileId')
                ->references('id')
                ->on('files');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('file_upload_locations');
    }
}

migrations_2017_12_08_224033_create_files_table.php

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateFilesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('files', function (Blueprint $table) {
            $table->increments('id');
            $table->string('filename');
            $table->integer('size');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('files');
    }
}