I reckon the best practice or standard way to achieve what you want (filtering/processing entity data before return as JSON to view), is to make use of serializer. You can consider using the Symfony Serializer Bundle: http://symfony.com/doc/current/components/serializer.html or the JMS Serializer: http://jmsyst.com/libs/serializer.
I myself recommend the JMS one, it has really good documentation, the definition is completely controlled through annotation, and a well-thought events system and a few feature I can list as follows:
-- Expose / Exclude specific entity attributes.
-- Create Virtual Property (attributes) for the entity once it converted to JSON. (So for example if you have Entity Product with attribute Price and you want to have a calculated field in the JSON object call taxPrice you can achieve it).
-- Access Control who can access which attributes/entities by categorising attributes to different groups.
-- Pre Serialization and Post Serialization events so you can tweak the data even further.
I can say the Serializer works so well with the Forest Bundle that once you get used to it, you can hardly write your project the different way :D.
