Updating models?

[igr]

What happens when a model gets updated?

It seems that Fridge fails to unfreeze existing data into a new structure. I see two cases here:

1. extended model (e.g., new field added): I expected this to work when a new field has a default value.
2. modified model (e.g., existing fields changes type etc): this is ok to fail.

One idea is to catch errors on load and then try to import data by first loading them into a generic dictionary/map (whatever is available on swift) and then manually importing data into the new, extended structure. This process could also give a mapping callback that could be used in case of modified fields (e.g. (fieldName, oldValue) -> newValue)

[vexy]

What happens when a model gets updated?

Unfortunately, Fridge fails to unfreeze a model that gets changed meanwhile. Reason for this unwanted behavior is internals of the freezing sub-system (file storage).

As this may lead to extra responsibility of the storage layer and increase complexity - which is tried to be avoided by "moving" these tasks to a upper layers - this is known limitation.

However, this may be sort of workaround solution:

• drop original model
• setup new (changed structure)
• freeze it again

Eventually, this may be wrapped into standalone function, named eg. swap(..) ?

[igr]

That's the thing, you can't drop an existing model as it contains production data.

[vexy]

True that, just dropping it carelessly would do more harm than good.

However, drop-and-replace actually came out as a possible solution to a completely different sphere/topic we're dealing here:
model/schema adjustments (on the go)

See below.

[igr]

As long as I think about this, it becomes more and more important :) An update of the model is something that often happens with production code. So I am thinking about the following (similar to e.g. flybase):

• there is additional storage (table) provided by the application that indicates application meta-data, like the model version.
• if the current version != stored version (update of the app) that is an indication that we need to upgrade the model.
• app will have upgrade methods: (Dictionary) -> Dictionary e.g. v2_add_field
• what we need from Fridge is the following:
  1. add the ability to read data in a generic collection, like a map/dictionary
  2. add the ability to map the dictionary into the model (this can be done in 2 steps: dictionary -> json -> model)

In short, it would be great if we can have some kind of load/save method that uses a generic collection.

[igr]

Something like loadRaw and freezeRaw methods;)

[igr]

I guess the following line would make be changed for the raw methods (from BSONConverter):

let realObject = try BSONDecoder().decode(T.self, from: rawBSONData)

i.e. this one in the Fridger:

let storedObject: T = try reader.read()

Instead of T in the 'raw' methods we might use Dictionary[String, Any]? Thinking out loud... its nothing urgent :)

[vexy]

Converting concrete type to Any would lead to type erasure problem and would lose all the benefits of generic approach 🫤

[igr]

Yeah, I was thinking of a separate method. Anyhow

let a: Dictionary<String, String> = try Fridge.unfreeze🪅🎉(objId)

does not work, even I have all the strings inside.

Nevermind! I just wanted to avoid carrying around all the model versions, but that might not be a that bad idea after all!

[igr]

We can close this question now :) Btw its not a Bug