MongoDB + MySQL with Doctrine 2. Example of listener to maintain redundant data (alternative to joins)

If you are developing an application with MongoDb, sooner or later you will face the need of solving a query that involves more than one collections (or a MongoDB collection with a MySQL table). In this case, an SQL join operation won’t be available.

If you have a small amount of data, you can perform two MongoDB/MySQL queries and then join the results at the application level, but – depending on your needs – that could be not feasible or turn out to be inconvenient. A possible better solution for that could be redundancy.

Example
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