debugging.md

Debug Helpers

NOTE: this functionality requires two additional gems to be available in the app:

• unparser
• method_source.

We usually describe policy rules using boolean expressions (e.g. A or (B and C) where each of A, B and C is a simple boolean expression or predicate method).

When dealing with complex policies, it could be hard to figure out which predicate/check made policy to fail.

The Policy#pp(rule) method aims to help debug such situations.

Consider a (synthetic) example:

def feed?
  (admin? || allowed_to?(:access_feed?)) &&
    (user.name == "Jack" || user.name == "Kate")
end

Suppose that you want to debug this rule ("Why does it return false?"). You can drop a binding.pry (or binding.irb) right at the beginning of the method:

def feed?
  binding.pry # rubocop:disable Lint/Debugger
  #...
end

Now, run your code and trigger the breakpoint (i.e., run the method):

# now you can preview the execution of the rule using the `pp` method (defined on the policy instance)
pry> pp :feed?
MyPolicy#feed?
↳ (
    admin? #=> false
    OR
    allowed_to?(:access_feed?) #=> true
  )
  AND
  (
    user.name == "Jack" #=> false
    OR
    user.name == "Kate" #=> true
  )

# you can also check other rules or methods as well
pry> pp :admin?
MyPolicy#admin?
↳ user.admin? #=> false