Fix #3872

[weiznich]

This commit changes some QueryFragment impls that previously assumed that the provided primary key consist only of a single column. The new implementation allows composite keys as well. In addition I also added two tests to cover these cases as well.

[AlexStorm1313]

@weiznich, out of curiosity could you elaborate a bit more on the naming of the DoNothingHelper trait? Functionality wise it looks good to me!

[weiznich]

There is no reasoning behind that name at all. I guess my thoughts were something like: It is somehow related to the DO NOTHING clause in ON CONFLICT statements and it's an internal helper trait -> DoNothingHelper. In the end it's just a trait that implements a helper to generate a workaround for the missing DO NOTHING support on mysql (by just saying id = id essentially).

That written: I'm open to change it to some better fitting name if someone comes up with a suggestion.

[Ten0]

It is somehow related to the DO NOTHING clause

Haha it's true that the name of the trait feels super weird like "wait what we need a trait to help us do nothing?" 😂
I guess just a tiny bit of doc comment on top of the trait would avoid this funny confusion 🙂

[weiznich]

Right, I didn't notice that the trait name might imply something else 🙈

I've pushed e8bc77a to include now explicitly in the name that it's about the DO NOTHING clause + added some lines of documentation.