Object union with optional parameter

[Treiblesschorle]

Hi,
I'm trying to get the union of two objects where one of the objects may be optional. i.e. undefined (one object is coming from data and the other from input and may or may not be there). I tried using object.union(objectA, objectB), however if one of the args is undefined the result is also undefined.
Does somebody have an idea how to solve that issue?

[srenatus]

You could use a rule, with one body per (defined, undefined) x (defined, undefined) pair, e.g.

a = data.foo
b = input.bar

union_maybe = object.union(a, b)

union_maybe = a {
  not b
}

union_maybe = b {
  not a
}

👉 Playground

[srenatus]

It wouldn't work like that function because get_or_default(data.foo, input.bar) wouldn't get evaluated unless both its operands were defined. That's how function calls work in OPA, I'm afraid.

We've been discussing a "defaulting dot" kind of operator, which would probably be of use, here, too: See open-policy-agent/opa#2345 for example.

[anderseknert]

While it won't work if the elements are in the "root" data location (i.e. data.x) as that would potentially be recursive you could use the object.get built-in function like object.get(data.x, "y", "my_default_value") for some cases like this.

[Treiblesschorle]

Thanks for the answers. So my initial problem is that the two objects I want the union of do actually contain further objects, i.e. I want to do a deep merge of the two objects. As the overall structure does not change, I tried the union on each of the nested objects. So a deep object merge would also solve my problem.

[Treiblesschorle]

For now I solved it with a bunch of nested object.get(data.x, "y", "my_default_value") calls. However, I think adding something like mentioned here open-policy-agent/opa#2345 would be nice. Thanks for the quick help guys :)

[anderseknert]

Indeed, that would be nice!