Pre-Condtion in `sqb/compile-schema` suggests only `columns` and `idents` is needed, but fails later for several other keys

[christoph-frick]

See https://github.com/walkable-server/walkable/blob/9621f1ca5fba0dbed06872c80ede5a7b646af3c7/src/walkable/sql_query_builder.cljc#L422

Due to the use of flatten-multi-keys later, also the following keys are basically mandatory for a schema:

• joins
• cardinality
• extra-conditions

While the first two most likely always exist anyways in any non-trivial example, extra-conditions might actually not be needed and the assert thrown later in flatten-multi-keys is way more confusing than just making them req-un for now?

[myguidingstar]

Great discovery! I think the above three should not be mandatory. flatten-multi-keys should only be applied after checking for non nil. Eg:

(flatten-multi-keys extra-conditions)

should become

(when extra-conditions
  (flatten-multi-keys extra-conditions))

are you willing to make another pr? :)

[christoph-frick]

What about putting them in the :or as empty maps?

https://github.com/walkable-server/walkable/blob/9621f1ca5fba0dbed06872c80ede5a7b646af3c7/src/walkable/sql_query_builder.cljc#L419

[myguidingstar]

that should work, too. Maybe that would save us from some null point exception headaches that I can't contemplate atm