Custom Lookup Chain

Action Policy's lookup chain is just an array of probes (lambdas with a specific interface).

The lookup process itself is pretty simple:

• Call the first probe;
• Return the result if it is not nil;
• Go to the next probe.

You can override the default chain with your own. For example:

ActionPolicy::LookupChain.chain = [
  # Probe accepts record as the first argument
  # and arbitrary options (passed to `authorize!` / `allowed_to?` call)
  lambda do |record, **options|
    # your custom lookup logic
  end
]

NullPolicy example

Let's consider a simple example of extending the existing lookup chain with one more probe.

Suppose that we want to have a fallback policy (policy used when none found for the resource) instead of raising an ActionPolicy::NotFound error.

Let's call this policy a NullPolicy:

class NullPolicy < ActionPolicy::Base
  default_rule :any?

  def any?
    false
  end
end

Here we use the default rule to handle any rule applied.

Now we need to add a simple probe to the end of our lookup chain:

ActionPolicy::LookupChain.chain << ->(_, _) { NullPolicy }

That's it!