How to Access a BelongsToMany Relationship Across Multiple Databases in Laravel?

Accessing a BelongsToMany Relationship Across Multiple Databases in Laravel

When working with models stored in different databases, you may encounter issues with Laravel's belongsToMany relationship when the pivot table is not in the same database as the target model. To overcome this, it is necessary to explicitly specify the pivot table's database name.

To do this, modify the belongsToMany relationship in the model that contains the pivot table as follows:

<code class="php">public function bs()
{
    $database = $this->getConnection()->getDatabaseName();
    return $this->belongsToMany('B', "$database.a_bs", 'a_id', 'b_id');
}</code>

This approach ensures that Laravel looks for the pivot table in the database corresponding to the model holding the relationship, thus resolving the issue.

However, if your project utilizes SQLite databases and you intend to run unit tests, additional considerations are necessary. SQLite requires attaching the target database to the testing connection to facilitate access to the pivot table. This can be achieved with the following code:

<code class="php">public function bs()
{
    $database = $this->getConnection()->getDatabaseName();
    if (is_file($database)) {
        $connection = app('B')->getConnection()->getName();
        $name = $this->getConnection()->getName();
        \Illuminate\Support\Facades\DB::connection($connection)->statement("ATTACH DATABASE '$database' AS $name");
        $database = $name;
    }
    return $this->belongsToMany('B', "$database.a_bs", 'a_id', 'b_id');
}</code>

It is important to be mindful of transaction usage. If the current connection employs transactions, the ATTACH DATABASE statement will fail. Conversely, if the related connection uses transactions, the resulting data will be concealed from the current connection.

To clean up after your attached database, you can either execute a migration rollback or truncate the tables while preserving their structure.

Other options include copying the attached database to a temporary file and replacing the source after the test or utilizing the easily accessible nature of SQLite database files.

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