Hi all,

I have an ExtraModel that I need to access in multiple apps. I am able to do that since ExtraModels are accessible. But my current implementation seems a little too hacky. I wondered if there is a better way to deal with that.

So, the problems I encountered:

- P1: ExtraModel is defined on a specific app.
- P2: ExtraModel needs to be linked to a specific model (Player, Group, Subsession) but I wasn't able to link it to a session-wide model (Error: Session/Participant is not defined). Thus, it is not possible to make calls like MyExtraModels.filter(code="abc", session=MySession) or MyExtraModels.filter(code="abc", participant=player.participant).

My workarounds:
P1: I was able to import the model (and relevant functions) from another app. For instance:

`from my_other_app import MyExtraModel, my_function1, my_function2`.

This works fine, but I am not sure it might create any future issues (like overwritten data or something).

P2: I created a function to first get all instances of a model and filters them out. This is probably too inefficient, and I can see it might create problems over time.

def filter_items_by_code(item_code=None):
    all_items = MyExtraModel.filter()
    filtered_items = [i for i in all_items if i.code == item_code]
    return filtered_chains

Okay, all works fine in this form, but am I missing something? Is there an easier way to deal with it?

ExtraModel is currently designed for use within an individual subsession, which is why it has those restrictions. I might extend it for use between apps/sessions/etc, but first I want to understand the use cases so I can come up with the right design. Can you tell me more about your use case? (what the MyExtraModel represents)

I am currently using it to create a "transmission chain": participants interact with each other one after the other. So typically one participant enter, get assigned to a "chain", possibly get some information from the previous person and once finished the chain is ready to accept another participants. So it is like a group with an order: P1 --> P2 --> P3 --> P4

My model is something like this:

class ChainModel(ExtraModel):
    subsession = models.Link(Subsession)
    chain_code = models.StringField() # A code to identify the chain. eg. ABCD
    available = models.BooleanField() # Whether the chain is ready to get the next person
    full = models.BooleanField(initial=False) # chain is full 
    complete = models.BooleanField(initial=False) # chain is completed
    
    p1 = models.Link(Player) 
    id1 = models.IntegerField()
    participant1 = models.StringField()
    finished1 = models.booleanField() # participant is finished
    information1 = models.LongStringField()
    # (I use a loop to generate one for other players in the "chain".

Then I have a bunch of functions like, get_my_chain, create_new_chain, add_player_to_chain, complete_current_player, drop_me_from_chain, get_status_of the chain etc. It also helped me to build a nice dashboard as an admin report. Here is my implementation: https://github.com/aseyq/chaining/tree/main

As it is very much like a group model, and I can probably use it and id_in_group can act like a participant's position/generation. But having this custom ExtaModel is flexible in the sense that I can build it on the fly at any point at a player level (as opposed to group/session level). I can assign a person to an existing group, remove a person who is not finished. 

I can possibly stay in a single app somehow, but having access to the chain in multiple apps makes it much easier and modular. Another reason is that I would like to turn it into an app or a package so others can re-use the same framework, which is very common in some fields.

Of course, for oTree, I have no idea if it makes sense to extend the ExtraModel to multiple apps in terms of cost/benefit. (And for my specific case I used the workarounds I mentioned, so it works alright) But I would think no matter what the use case is, having that flexibility would simplify lots of participant/session var implemenations with complex information. (Like having treatment/session level custom tables in z-Tree). The necessary structure seems to be already there and accessible across the apps.

Thank you for the description. I’ll look at extending ExtraModel to the use case you describe.

Thank you so much, Chris. I am at your service if you need any feedback or if something I can help with comes up.