# db-subclass-to-dto

This is a workspace for creating two exercises.

One exercise is about exploring a repository using prompts for a coding agent to create Mermaid and PlantUML diagrams. We're focusing on class diagrams and sequence diagrams to get an idea of what is going on. We use the starting point of the second exercise for this.

The second exercise is factoring out domain objects, domain entities, from a structure that is previously subclass-based. So say you use a structure like active records or Entity Framework where you have an active record class and your domain class subclasses from the active record class which gives you maybe some nice things like find by an attribute. And then you can quite quickly — so these are straight-offs — you can quite quickly structure an application while it's also persisted. So you can say, "I want to have a customer, I want to have an order. And oh yes, I'm doing this for a webshop." So you have objects, relations, attributes, and you can all easily save them in your database. And this is fine when your application is small and you're just starting out. It allows you to very quickly scaffold an application. But it can become painful when your data doesn't have the right shape for it. So performance is slow. So at some point as your application grows, you probably want to be more deliberate in how you start.

The second exercise is factoring out domain classes from a database inheritance structure. So we start with a couple of domain classes and some relations. Then we pre-deliver some classes that we already created and we demo it. As we have some domain classes, we create a mapping for writing and reading a domain class. And we make sure that running the existing tests against the fake database that we have for the exercise is really slow. The sequence diagrams that we make in the first exercise will come in handy. Because in order to do this, you do start wondering how does this web application find a client? How can we see clients? How can we create an order? And what happens once the flow through the application as we do this? You sort of need to understand this to choose the kind of mapping that you use. So we are going to reuse the analysis we did in the first exercise in the second exercise.