First of all I have to thank percious. He is incredibly reliable and helpful. There is actually a bug in sprox that makes him threat one-to-many relationships as one-to-one relationships and makes it show a single selection field instead of a multiple selection field. This can be avoided changing the field type to sprox.widgets.PropertyMultipleSelectField but percious has been so kind to fix it on the fly while I was testing the problem for him and now sprox correctly detects the field type by default.
Bad enough there is a big problem with Elixir. As Sprox probably creates internal instances of the Entity you pass to him this causes an undesidered behaviour. When using SQLAlchemy, until you add the object to the session it won’t be saved on the database, but with Elixir creating an object means saving it on the db and this results in having multiple empty entities saved on db each time you open a forum generated with Sprox. If you have any required field in your entity your application will crash as it won’t be able to save it.
In the end I had to switch back from Elixir to DeclarativeBase for my application and everything worked fine