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I went through the docs and couldn't find a way to check whether an attribute exists or not.
Is this expression valid?
`expr: request.resource.attr.roles == null`

Cerbos uses Google cel language. This might help. <https://github.com/google/cel-go#macros|https://github.com/google/cel-go#macros>

Thanks. Something like this seems to work:

`expr: has(request.resource.attr.roles) == false`

I was thinking that there should be a better way to write it.

Expr: !has(.....) try that. I cannot remember off-hand but that might work.

The `!` operator should work. You can try out expressions in our <https://docs.cerbos.dev/cerbos/latest/cli/cerbos#repl|REPL>

```-&gt; :let x = {"foo": "bar"}
x = {
  "foo": "bar"
}

-&gt; has(x.foo)
_ = true

-&gt; !has(x.foo)
_ = false

-&gt; !has(x.bar)
_ = true```

image.png

Thanks for introducing the REPL by the way.

Hmm.. that's odd. Could you try writing it like this:
```- expr: &gt;
      !has(request.resource.attr.roles)```

<@U05LSJLMN1G> `!` has special meaning in YAML, so in your case `!has` gets interpreted as YAML tag, and not as CEL. Check out the YAML spec for more info: <https://yaml.org/spec/1.2-old/spec.html#id2784064>

Yes. You're completely right <@U04P0S61Q65>

Your workaround works perfectly <@U02874JL5D3>

I love learning new things when working with Cerbos.

I will be writing lots of blog posts soon.