Entity Type

The Entity type is a struct that holds the events: EntityEvents<EntityEvent> field. Mutations of the entity append events to this collection. The Entity is (re-)constructed from events via the TryFromEvents trait. Other than the events field additional fields can be exposed and populated during hydration.

#![allow(unused)]
fn main() {
extern crate es_entity;
extern crate sqlx;
extern crate serde;
use serde::{Deserialize, Serialize};
use es_entity::*;
es_entity::entity_id! { UserId };
#[derive(EsEvent, Debug, Serialize, Deserialize)]
#[serde(tag = "type", rename_all = "snake_case")]
#[es_event(id = "UserId")]
pub enum UserEvent {
    Initialized { id: UserId, name: String },
    NameUpdated { name: String },
}
pub struct NewUser { name: String }
impl IntoEvents<UserEvent> for NewUser {
    type IdSource = FromRepo;
    fn into_events(self, builder: EntityEventsBuilder<UserEvent, FromRepo>) -> EntityEvents<UserEvent> {
        let id = builder.id();
        builder.initial_events([UserEvent::Initialized {
            id,
            name: self.name,
        }])
    }
}
// `#[derive(EsEntity)]` generates `UserHydrator`
// for hydrating in the `TryFromEvents` trait.
// The hydrator is private to the module that declares the entity;
// widen it with e.g. `#[es_entity(hydrator = "pub(crate)")]`.
#[derive(EsEntity)]
pub struct User {
    pub id: UserId,
    pub name: String,

    // The `events` container - mandatory field.
    // Basically its a `Vec` wrapper with some ES specific augmentation.
    events: EntityEvents<UserEvent>,

    // Marker if you use a name other than `events`.
    // #[es_entity(events)]
    // different_name_for_events_field: EntityEvents<UserEvent>
}

impl User {
    pub fn update_name(&mut self, new_name: impl Into<String>) -> Idempotent<()> {
        let new_name = new_name.into();
        // The idempotency_guard macro is a helper to return quickly
        // if a mutation has already been applied.
        // It is not mandatory but very useful in the context of distributed / multi-thread
        // systems to protect against replays.
        idempotency_guard!(
            self.events.iter_all().rev(),
            // If this pattern matches return Idempotent::AlreadyApplied
            already_applied: UserEvent::NameUpdated { name } if name == &new_name,
            // Stop searching here
            resets_on: UserEvent::NameUpdated { .. }
        );

        self.name = new_name.clone();
        self.events.push(UserEvent::NameUpdated { name: new_name });

        Idempotent::Executed(())
    }
}

// Any EsEntity must implement `TryFromEvents`.
// This trait is what hydrates entities after loading the events from the database
impl TryFromEvents<UserEvent> for User {
    fn try_from_events(events: EntityEvents<UserEvent>) -> Result<Self, EntityHydrationError> {
        let mut hydrator = User::hydrator();
        for event in events.iter_all() {
            match event {
                UserEvent::Initialized { id, name } => {
                    hydrator.id(*id).name(name.clone());
                }
                UserEvent::NameUpdated { name } => {
                    hydrator.name(name.clone());
                }
            }
        }
        hydrator.build(events)
    }
}

#[cfg(test)]
mod tests {
    use es_entity::testing::*;

    use super::*;

    fn fresh_user() -> User {
        User::hydrate_from_new(UserId::new(), NewUser { name: "Willson".to_string() })
            .expect("Could not create user")
    }

    #[test]
    fn update_name() {
        let mut user = fresh_user();

        // There are no new events to persist after hydrating
        assert!(!user.events().any_new());

        let new_name = "Gavin".to_string();
        assert!(user.update_name(new_name.clone()).did_execute());
        assert_eq!(user.name, new_name);

        // Mutations are expected to append events
        assert!(user.events().any_new());
    }
}
}

See Testing for building a User like this one in unit tests.