Include a nested relation to one schema, without modifying the rest of the functions that use it

Basically I would like to add a new nested relation to an schema, that I already use in other services, without this implying a change to the rest of the functionalities that use it for serialization. Since I work with sqlalchemy, if I include the new relation, new unwanted statements appears everywhere, since the ORM tries to fill it. I would need something like adding the relation, and specifying that be only included if I specify it. I seen this kind of problem resolved in other libraries using groups annotations, so in this case, the relation is added, annotated with a new group, and them call serialize using the existing groups plus the new one.

Note: Create a new schema is not a good solution since the affected schema is deep in the relation, so I would have to create all it's fathers too.

The issue names no files, tests, or entry points, so begin by locating schema serialization and nested-relation handling in the repository. Define how an optional group or equivalent would be declared and selected, then add coverage showing the relation is excluded by existing serialization and included when the new option is requested.