[7.x] Configure Schema on migration instantiation

[antonkomarev]

This PR adds to abstract Migration class $schema property with configured connection from Migration's getConnection() method.

Motivation

Migration has getConnection() method which used inside of the Migrator class to determine if schema transactions are supported:

protected function runMigration($migration, $method)
{
    $connection = $this->resolveConnection(
        $migration->getConnection()
    );

    $callback = function () use ($migration, $method) {
        if (method_exists($migration, $method)) {
            $this->fireMigrationEvent(new MigrationStarted($migration, $method));

            $migration->{$method}();

            $this->fireMigrationEvent(new MigrationEnded($migration, $method));
        }
    };

    $this->getSchemaGrammar($connection)->supportsSchemaTransactions()
        && $migration->withinTransaction
                ? $connection->transaction($callback)
                : $callback();
}

But when migration is running Schema facade is being used and it doesn't care about getConnection() method of the migration. Migrator checks connection and enables transactions, but in fact migrations could be executed on another connection.

To respect database connection, instead of this:

Schema::create('users', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->timestamps();
});

Schema::dropIfExists('users');

We should write this:

Schema::connection($this->getConnection())->create('users', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->timestamps();
});

Schema::connection($this->getConnection())->dropIfExists('users');

New Implementation

$this->schema->create('users', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->timestamps();
});

$this->schema->dropIfExists('users');

Similar approach implemented in Laravel Telescope, but there it doesn't get connection name from getConnection() method, what means that Migrator wouldn't determine correctly if schema transactions are supported.

[taylorotwell]

No plans to change this right now.

[antonkomarev]

@taylorotwell this solution is not how you see it should work or what?