Feature request: encoding foreign key arity

[TeofilC]

When we have a foreign key in Rel8, we don't really know if it's one-to-one or one-to-many.
In fact we don't really distinguish foreign keys from normal values at all.

It would be nice to encode this information somehow. I don't have a concrete design. But it would really help if I could just tell from a glance that a certain line will always succeed or always give exactly one result.

This is always a source of uncertainty when writing or reading queries.

[ocharles]

This has come up at various points I think, but I'm not entirely convinced. I'm mostly worried that the typing overhead is going to become annoying, or impair inference. I could be wrong though - I've never tried! My general advice is to push this in to naming. I encourage FKs be represented by "join functions", like projectOwner :: Project Expr -> Query (User Expr). Maybe this idea could be extented with a naming convention to indicate cardinality: projectOwner1 or something, vs projectCollaboratorsN. Certainly something to think about here.

[TeofilC]

A naming convention is a good idea!

I was thinking as well, with foreign keys, you know that the query will always succeed, so we could theoretically have projectOwner :: Project Expr -> User Expr. I'm not sure if this makes any sense at all wrt to the implementation, but this would be a nice way to express that there is always exactly one User.