Add selector method capabilities to selectors

[danieldiamond]

resolves #4821

Description

Add selector method in order to be able to define a selector with another selector definition.

To Do

• Prevent circular dependencies
• unit test to ensure circular dependency check works
• unit test to confirm selector method has expected behaviour

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change

[danieldiamond]

Initial attempt, seems to be working but not such a fan of injecting the selector_dict through the class methods, particularly as we're updating it (hence the deepcopy). Ideally, this would be an instantiated class to lean on self.selector_dict or similar.

[danieldiamond]

@jtcohen6 any capacity this month for the team to review?

[gshank]

This looks pretty good. I see that you have a checkbox unchecked for circular dependencies though. Do you have an idea of how to handle that? It might be tricky to handle robustly, but it would be good to create a test case with circular dependencies and see what error occurs. Catching the error at a higher level would probably be fine (if possible). A python error for recursiveness would be not optimal...

[gshank]

Oh, also we now use 'changie' for the changelog, so the CHANGELOG.md should not be changed from main. See: https://github.com/dbt-labs/dbt-core/blob/main/CONTRIBUTING.md#adding-changelog-entry

[danieldiamond]

I see that you have a checkbox unchecked for circular dependencies though. Do you have an idea of how to handle that?

I was hoping to get some guidance on this. Is this something you can help with @gshank ?
Something as simple as checking if the selector key already exists then error out as "already defined at root" or similar

[gshank]

That part will probably be a bit tricky. I think we'd have to keep a list of already encountered selectors in this branch of the tree and check if this selector was encountered higher up. I haven't touched this code in a while, so I'd have to pull it apart a bit further to figure out where that would have to go. For recursive functions, I generally just pass along a list of names that I check...

[danieldiamond]

@gshank any chance I can get some help to get this over the line? Please and thank you in advance!

[gshank]

I won't have time today, but I'll try to find some time to take a look this week.

[gshank]

I did some testing and it looks like we don't end up with recursion, we just get "Existing selector definition for not found." Which is pretty innocuous.