Change database in CakePHP 3 based on request data

Application.php

<?php
/*
 * Allow development users to switch between databases
 * ...add to \App\Application::middleware
 * */

$middlewareQueue->add(function (ServerRequest $request, Response $response, $next) {

    $isPreviouslyAliased = false;

    // restore the alias `db_alias`
    if (Configure::restore('db_alias', '_cake_session_')) {
        $dbAliasStored = Configure::read('App.db_alias');
        $isPreviouslyAliased = !empty($dbAliasStored);
    }

    // post in a `db_alias` from a form or set within other controller logic
    $dbAliasRequested = $request->getData('db_alias');
    $isRequested = (null !== $dbAliasRequested);
    // a test, implementing only in dev environment
    $isDevelopment = Configure::read('App.environment') === 'development';

    if ($isDevelopment && ($isPreviouslyAliased || $isRequested)) {

        // presidence given to request data, before any stored alias.
        if ($isRequested) {
            $dbAliastoUse = $dbAliasRequested;
        } elseif ($isPreviouslyAliased) {
            $dbAliastoUse = $dbAliasStored;
        } else {
            $dbAliastoUse = 'stage';
        }

        // create a new dynamic db configuration, cloning a single `default` db config from config/app.php
        $defaultDbConfig = ConnectionManager::getConfig('default');
        $aliasedDbConfig = $defaultDbConfig;
        $aliasedDbConfig['database'] = $dbAliastoUse;
        $aliasedDbConfig['name'] = $dbAliastoUse;
        ConnectionManager::setConfig($dbAliastoUse, $aliasedDbConfig);

        $dbConnections = ConnectionManager::configured();
        if (in_array($dbAliastoUse, $dbConnections)) {
            // alias it to the `default` db config
            ConnectionManager::alias($dbAliastoUse, 'default');
            // store the `db_alias` alias for future non-async requests
            Configure::store('db_alias', '_cake_session_', ['App.db_alias' => $dbAliastoUse]);
        }

    }

    return $next($request, $response);
});

// In a logout or similar event listener, drop the db alias
if (Configure::restore('db_alias', '_cake_session_')) {
    $dbAliasStored = Configure::read('App.db_alias');
    if (!empty($dbAliasStored)) {
        ConnectionManager::dropAlias($dbAliasStored);
    }
}

Based on an article by @markstory