Using Marshmallow `Method` Fields for Complex Nested Schemas in Flask

Introduction I made an habit tracking journal app where users can write entries about their habit loops. A habit loop consists of a trigger or cue, a behavior, and a result. For privacy reasons and a clean database design, triggers are related to a user, while behaviors are available to all users. Users can write entries that have one trigger and one behavior. A user's entries are still private to them through the trigger. I wanted an API response that returns all of a user's data with nested schemas. Due to the model relationships, there can be two hierarchical views of the same data: User > Trigger > Behaviors > Entries User > Behavior > Triggers > Entries So the API response needs to: List triggers with their related behaviors and only the entries that belong to both the trigger and behavior . List behaviors with their related triggers and those same filtered entries. Include user-specific filtering to only show relevant triggers and behaviors: a user should not see behaviors, trigg