Multi-tenancy Angel middleware

middleware.dart

import 'dart:async';
import 'package:angel_framework/angel_framework.dart';
import 'package:mongo_dart/mongo_dart.dart';

/// Chooses the database associated with the [tenantId], and injects it into the [RequestContext].
///
/// @[Header] annotation is used to parse out the `x-tenant` header, if it is not present, a 400 is thrown.
Future<bool> chooseDatabase(@Header('x-tenant') tenantId, Db db, RequestContext req) async {
  // Assuming that you have a collection that maps tenant ID's to collection names:
  var mapping = await db.collection('tenants').findOne(where.id(new ObjectId.fromHexString(tenantId)));
  var connectionString = mapping['connection_string'];
  var scopedDb = new Db(connectionString);
  await scopedDb.open();
  
  // Once we inject this as a dependency, it can be passed throughout the duration of the request.
  req.inject(Db, scopedDb);
  return true;
}

routes.dart

import 'dart:async';
import 'package:angel_framework/angel_framework.dart';
import 'package:mongo_dart/mongo_dart.dart';
import 'middleware.dart';

/// Add some pseudo-code routes to the server...
Future configureServer(Angel app) async {
  // Call `chain` to apply a middleware 
  app.chain(chooseDatabase).get('/leads', (Db db, RequestContext req) {
    // Since we injected [Db], the value of `db` here is the `scopedDb` from earlier.
    // If each tenant has their own database, then the 'leads' collection will pertain only to that tenant.
    var leadsCollection = db.collection('leads');
    return leadsCollection.findAll();
  });
}