Feat: optional where clause in relations

[Angelelz]

This PR will close #1437, and will close #1051, will address part of #674 and part of #821.

Upon successful merging of this PR, a new where option will be added to the relation configuration. The most obvious use case is to better support for polymorphic associations when you'll like to associate a table with another one, only when a condition is met:

export const usersRelations = relations(users, ({ many }) => ({
  comments: many(comments, {
    where: eq(comments.commentType, 'user')
  }),
}));

The new where configuration option is also available in the one relation.
Please note that adding a where condition in the one relation will mark that field as nullable even if the column is not nullable, as we don't have a way of knowing at compile time if any row will be returned from the DB.

[raikusy]

Any update on this?

[andrewdoro]

Hey just saw this, it would be really nice to have this merged

[TaylorFacen]

Hi, this looks amazing. Would love to have this merged in.

[ryanimpey]

Is there any update on this? Would love to have this in the next release!

[faribauc]

+1 Yes please!

[kenn]

Folks, relational API v2 is discussed at #2316