I think other merge "modes" would be really useful.

How would you feel about something like this?

generators:
- merge:
  mode: left-join

I like that idea. Would be great to add more modes in the future (e.g. inner-join , left-outer-join ...)

Wondering what the current merge mode should be called. It works like a left-join that strictly only allows unique keys.

Good question. left-unique?

When I wrote the original merge generator code, I was writing for a specific internal use case with the expectation that other modes would eventually be added. I tried to use a SQL analogy for our use case, but it quickly broke down. :-)

Whatever name makes it most clear for the user is I think best.

While this is a really a very unique case but this is exactly what I am stuck with right now .

inner-join would help in not generating application without secondary generator param values.
Actually, when a base param set is not matched in the secondary param sets, if you use a param from the secondary generator in the template, it will be replaced by <no value> which is definitely wrong.

Maybe merge should be left as-is and have a join plugin ? I mean merge allows to override param of previous generator while join would not.