Teach createOrUpdatePartial to use non-id fields

[stephenh]

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.