Guidelines.md

+## Where to put code
+
+We have the following conventions for deciding when to put code in a `Controller`/`Service`/helper method:
+
+- A small number of lines can go directly into the controller method.
+- When a controller method does multiple things or becomes too long it should be split into helper methods.
+- When code is duplicated or can be reused it should be moved from the controller to a service.
+- Data retrieval from the database in implemented in the repository class unless it can not be done with simple queries or queryDSL anymore.
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