How to assert deposited event in pallet call?

Question

In testing my pallet, I have some function that stores something and deposits an event.

    #[pallet::event]
    #[pallet::generate_deposit(pub(super) fn deposit_event)]
    pub enum Event<T: Config> {
        ThingStored(thing<T>, T::AccountId),
    }

    #[pallet::weight(10_000 + T::DbWeight::get().writes(1))]
    pub fn create_thing(origin: OriginFor<T>) -> DispatchResult {
        let creator = ensure_signed(origin)?;
        let id = 1;

        let thing = Thing {
            id,
            creator: creator.clone()
        };

        // Update storage.
        <Things<T>>::insert(id, thing.clone());

        // Emit an event.
        Self::deposit_event(Event::ThingStored(thing, creator.clone()));
        Ok(())
    }

In my pallet tests, I am calling the function and would like to assert that the event deposited contains the correct data.

#[test]
fn it_works_for_default_value() {
    new_test_ext().execute_with(|| {
        // Dispatch a signed extrinsic.
        assert_ok!(ThingsModule::create_thing(Origin::signed(1)));
        let event = <frame_system::Pallet<Test>>::events().pop()
            .expect("Expected at least one EventRecord to be found").event;
        // How can I access the `Thing` object from the event? I'll also then use it to read from storage
        // assert_eq!(ThingsModule::things(thing.creator).unwrap().creator, 1);
    });
}

Answer 1

The FRAME System pallet has two helper functions for this:

/// Assert the given `event` exists.
#[cfg(any(feature = "std", feature = "runtime-benchmarks", test))]
pub fn assert_has_event(event: T::Event) {
    assert!(Self::events().iter().any(|record| record.event == event))
}

/// Assert the last event equal to the given `event`.
#[cfg(any(feature = "std", feature = "runtime-benchmarks", test))]
pub fn assert_last_event(event: T::Event) {
    assert_eq!(Self::events().last().expect("events expected").event, event);
}

You can see a lot of examples of these functions being used by searching the Substrate codebase, but here is one simple example:

System::assert_last_event(ProxyEvent::ProxyExecuted { result: Ok(()) }.into());

Note the .into() at the end which converts a Pallet specific event to a more general T::Event.