Teach createOrUpdatePartial to use non-id fields

It'd be neat if the `upsert`-capabilities of `createOrUpdatePartial` could use unique columns like `code` to find an existing row.

Also maybe "unique per parent", i.e. a `Product` parent with `ProductDimension` children, but there should be only one `ProductDimension` for a given `type` for each `Product`.

With the combination of `config.addRule(uniquePerParentAnd("product", "type"))`, we could take `saveProduct({ id: "pr:1", dimensions: [{ type: ..., value: ... }] })` and update existing dimensions-of-that-type.