Cascade DELETEs in the database

[gtfierro]

I'm pretty sure the issue is that if I create library A with templates, and then I create library B which depends on A through B's templates, and the nI delete library A, I need to decide what happens to A's templates and B's templates' dependencies on A's templates. Right now (outside of this PR) I think we leave dangling references, which causes exceptions in SQLalchemy when you interact with that table after messing with the dependency structure.

[TShapinsky]

@gtfierro I just rebased which mostly required changes around of exceptions, cause your adding exceptions PR went through to develop. Let me know if anything looks messed up.

[TShapinsky]

Documenting what appears to be the desired functionality so that it can be tested.

On Model Delete

• Delete associated shape collection

On Library Delete

• Delete shape collection
• Delete all templates owned by library
• Delete all dependency relationships associated with deleted templates.

Standing issues:

• What happens when you delete a library which has templates depended on by another libraries templates.

[gtfierro]

This looks great, @TShapinsky ! Thanks for doing the merge. I just wrote a couple unit tests to verify the behavior under different conditions.

I decided to see what would happen if we had a default CASCADE between dependencies.
Currently, the test_cascade_delete_dependency_multi_library test fails because it specifies that if I delete the library my template is dependent on, then that should also delete the whole library (this is the behavior I am proposing). What do you think of this behavior?

[TShapinsky]

This looks great, @TShapinsky ! Thanks for doing the merge. I just wrote a couple unit tests to verify the behavior under different conditions.

I decided to see what would happen if we had a default CASCADE between dependencies. Currently, the test_cascade_delete_dependency_multi_library test fails because it specifies that if I delete the library my template is dependent on, then that should also delete the whole library (this is the behavior I am proposing). What do you think of this behavior?

So if a library is deleted it should cascade delete all libraries which have dependent links to that library.

I think that makes sense. We should make sure that the deletes don't accidentally cascade the other direction. I haven't looked at your new tests yet so you may have already addressed that. Unless we want to go through the process of rebuilding dependencies for all libraries anytime one is imported I think this is the right call.

One other thought. If this isn't already the case, we should make sure that you can hotswap a library without destroying the entire dependency tree. For Example: if you overwrite a library it should rebuild the dependencies but not cascade delete everything. This could probably be a different issue.