On Delete item cascading

[vlad-ed-git]

• Test deleting a contact -> what happens to the client holding it
• Test deleting a client -> what happens with the contract or project referencing it
• Test deleting a contract -> what happens to the project referencing it

[clstaudt]

The logic is straightforward: As long as a Contact is required by a Client, then it cannot be deleted. I think it's clear that the user does not want "cascading" deletion.

So, test for len(contact.invoicing_contact_of) > 0, if true do not allow deletion.

The implementation in the current MVI architecture is not straightforward though. e.g.: Where should this test happen? In the intent or the data source?

I chose the intent layer and implemented it for the deletion of contacts. @vlad-ed-git Please review my implementation.

[vlad-ed-git]

@clstaudt

The View

• builds Ui,
• reacts to data changes (by updating the Ui)
• listens for events and forwards them to the Intent

The Intent

• receives events
• if some data is affected by the event, figure out which data source corresponds to that data
  - transforms the event data to the data format required by the data source
  - transform returned data source data to the data format required by the Ui
• else, no data is affected by the event, handle the event (often using a util class).
  an example of this is sending invoices by mail.

The Model (a.k.a data layer)

• define the entity
• define the entity source (file, remote API, local database, in-memory cache, etc)
  - if a relational database is used, define the entity's relationship to other entities
  - maintain the integrity of that relation (conflict strategies for insert operations are defined here, and integrity errors are thrown here, for example)
• define classes that manipulate this source (open, read, write, ....)

• As you go outer in layers (the outmost layer is the Ui, the innermost is the data layer), communication can occur down_ward across layers, and horizontally (for lack of a better word) BUT a layer cannot skip the layer directly below it. This is to say:

  * Data sources can communicate with each other. Thus `ClientDatasource.delete_client` can call. `ContractDatasource.get_contract ` for example.
  
  * Intents can communicate with each other, and with any data source. Thus `ClientIntent` can call `ContractIntent` or  `ContractDatasource` for example.
  

  • The Ui can communicate with any intent (though often the Ui is tied to only a single intent, and the intent can instead call the 'other' intent). But it cannot communicate with a data source -> this would violate the do not skip layers rule.

• An inner layer cannot have a dependency on the layer above it. Thus an intent cannot instantiate a Ui class, and a data source cannot instantiate an Intent class.

[vlad-ed-git]

• The above tasks fall under entity integrity management, and should thus be done in the data source.
  @clstaudt

If you are wondering how the intent would know if a delete did not happen because of some other error or because the object being deleted is depended on by another, this is where error_codes typically come in.

The IntentResult can define an error code (Error codes might be wrapped in enums for readability).
So in the case where a deletion fails because the entity being deleted is being used by another entity, the data source will return IntentResult with was_intent_sucessful = False, log_message="...IntegrityError, attempting to delete an entity referenced by another ...", error_code = ErrorCodes.IntegrityError

[clstaudt]

Thanks for clarifying.

• transforms the event data to the data format required by the data source
• transform returned data source data to the data format required by the Ui

Somewhat missing from the picture is the role of an ORM (Object Relational Mapper). SQLModel already abstracts the concrete database implementation, something that would otherwise be handled by the DataSource. And it seems to me that with SQLModel the data format required by the UI and the data source is often exactly the same. So not much transformation going on in practice.

[clstaudt]

fastapi/sqlmodel#533 (comment)