diff --git a/src/aggregate.rs b/src/aggregate.rs
index 22f2461..6477430 100644
--- a/src/aggregate.rs
+++ b/src/aggregate.rs
@@ -3,6 +3,7 @@ use std::marker::PhantomData;
 
 use crate::decider::{Decider, EventComputation, StateComputation};
 use crate::saga::{ActionComputation, Saga};
+use crate::Identifier;
 
 /// Event Repository trait
 ///
@@ -316,7 +317,11 @@ where
         }
     }
     /// Handles the command by fetching the events from the repository, computing new events based on the current events and the command, and saving the new events to the repository.
-    pub async fn handle(&self, command: &C) -> Result<Vec<(E, Version)>, Error> {
+    pub async fn handle(&self, command: &C) -> Result<Vec<(E, Version)>, Error>
+    where
+        E: Identifier,
+        C: Identifier,
+    {
         let events: Vec<(E, Version)> = self.fetch_events(command).await?;
         let mut current_events: Vec<E> = vec![];
         for (event, _) in events {
@@ -336,7 +341,11 @@ where
         &self,
         current_events: &[E],
         command: &C,
-    ) -> Result<Vec<E>, Error> {
+    ) -> Result<Vec<E>, Error>
+    where
+        E: Identifier,
+        C: Identifier,
+    {
         let current_state: S = current_events
             .iter()
             .fold((self.decider.initial_state)(), |state, event| {
@@ -361,7 +370,11 @@ where
                     .iter()
                     .map(|(e, _)| e.clone())
                     .collect::<Vec<E>>(),
-                initial_events.clone(),
+                initial_events
+                    .clone()
+                    .into_iter()
+                    .filter(|e| e.identifier() == command.identifier())
+                    .collect::<Vec<E>>(),
             ]
             .concat();
 
diff --git a/src/lib.rs b/src/lib.rs
index 82e38ec..6890301 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -340,3 +340,10 @@ pub enum Sum<A, B> {
     /// Second variant
     Second(B),
 }
+
+/// Identify the state/command/event.
+/// It is used to identify the concept to what the state/command/event belongs to. For example, the `order_id` or `restaurant_id`.
+pub trait Identifier {
+    /// Returns the identifier of the state/command/event
+    fn identifier(&self) -> String;
+}
diff --git a/tests/aggregate_combined_test.rs b/tests/aggregate_combined_test.rs
index 0fb5c7c..fc4f711 100644
--- a/tests/aggregate_combined_test.rs
+++ b/tests/aggregate_combined_test.rs
@@ -8,6 +8,7 @@ use fmodel_rust::aggregate::{
 };
 use fmodel_rust::decider::Decider;
 use fmodel_rust::saga::Saga;
+use fmodel_rust::Identifier;
 
 use crate::api::{
     CancelOrderCommand, CreateOrderCommand, CreateShipmentCommand, OrderCancelledEvent,
@@ -16,7 +17,6 @@ use crate::api::{
 };
 use crate::application::{
     command_from_sum, event_from_sum, sum_to_command, sum_to_event, AggregateError, Command, Event,
-    Id,
 };
 
 mod api;
@@ -44,7 +44,7 @@ impl EventRepository<Command, Event, i32, AggregateError> for InMemoryEventRepos
             .unwrap()
             .clone()
             .into_iter()
-            .filter(|(event, _)| event.id() == command.id())
+            .filter(|(event, _)| event.identifier() == command.identifier())
             .collect())
     }
 
@@ -75,7 +75,7 @@ impl EventRepository<Command, Event, i32, AggregateError> for InMemoryEventRepos
             .unwrap()
             .clone()
             .into_iter()
-            .filter(|(e, _)| e.id() == event.id())
+            .filter(|(e, _)| e.identifier() == event.identifier())
             .map(|(_, version)| version)
             .last())
     }
@@ -102,7 +102,12 @@ impl StateRepository<Command, (OrderState, ShipmentState), i32, AggregateError>
         &self,
         command: &Command,
     ) -> Result<Option<((OrderState, ShipmentState), i32)>, AggregateError> {
-        Ok(self.states.lock().unwrap().get(&command.id()).cloned())
+        Ok(self
+            .states
+            .lock()
+            .unwrap()
+            .get(&command.identifier().parse::<u32>().unwrap())
+            .cloned())
     }
 
     async fn save(
@@ -114,7 +119,7 @@ impl StateRepository<Command, (OrderState, ShipmentState), i32, AggregateError>
         self.states
             .lock()
             .unwrap()
-            .insert(state.id(), (state.clone(), version + 1));
+            .insert(state.0.order_id, (state.clone(), version + 1));
         Ok((state.clone(), version))
     }
 }
diff --git a/tests/aggregate_test.rs b/tests/aggregate_test.rs
index dad6fc5..6a8861c 100644
--- a/tests/aggregate_test.rs
+++ b/tests/aggregate_test.rs
@@ -6,6 +6,7 @@ use fmodel_rust::aggregate::{
     EventRepository, EventSourcedAggregate, StateRepository, StateStoredAggregate,
 };
 use fmodel_rust::decider::Decider;
+use fmodel_rust::Identifier;
 
 use crate::api::{
     CancelOrderCommand, CreateOrderCommand, OrderCancelledEvent, OrderCommand, OrderCreatedEvent,
@@ -43,7 +44,7 @@ impl EventRepository<OrderCommand, OrderEvent, i32, AggregateError>
             .unwrap()
             .clone()
             .into_iter()
-            .filter(|(event, _)| event.id() == command.id())
+            .filter(|(event, _)| event.identifier() == command.identifier())
             .collect())
     }
 
@@ -74,7 +75,7 @@ impl EventRepository<OrderCommand, OrderEvent, i32, AggregateError>
             .unwrap()
             .clone()
             .into_iter()
-            .filter(|(e, _)| e.id() == event.id())
+            .filter(|(e, _)| e.identifier() == event.identifier())
             .map(|(_, version)| version)
             .last())
     }
@@ -100,7 +101,12 @@ impl StateRepository<OrderCommand, OrderState, i32, AggregateError>
         &self,
         command: &OrderCommand,
     ) -> Result<Option<(OrderState, i32)>, AggregateError> {
-        Ok(self.states.lock().unwrap().get(&command.id()).cloned())
+        Ok(self
+            .states
+            .lock()
+            .unwrap()
+            .get(&command.identifier().parse::<u32>().unwrap())
+            .cloned())
     }
 
     async fn save(
diff --git a/tests/api/mod.rs b/tests/api/mod.rs
index c7d2adb..a47aadc 100644
--- a/tests/api/mod.rs
+++ b/tests/api/mod.rs
@@ -2,6 +2,8 @@
 // ############################ Order API ############################
 // ###################################################################
 
+use fmodel_rust::Identifier;
+
 /// The state of the Order entity
 #[derive(Debug, Clone, PartialEq)]
 pub struct OrderState {
@@ -47,14 +49,13 @@ pub struct CancelOrderCommand {
     pub order_id: u32,
 }
 
-/// Provides a way to get the id of the Order commands
-impl OrderCommand {
+impl Identifier for OrderCommand {
     #[allow(dead_code)]
-    pub fn id(&self) -> u32 {
+    fn identifier(&self) -> String {
         match self {
-            OrderCommand::Create(c) => c.order_id.to_owned(),
-            OrderCommand::Update(c) => c.order_id.to_owned(),
-            OrderCommand::Cancel(c) => c.order_id.to_owned(),
+            OrderCommand::Create(c) => c.order_id.to_string(),
+            OrderCommand::Update(c) => c.order_id.to_string(),
+            OrderCommand::Cancel(c) => c.order_id.to_string(),
         }
     }
 }
@@ -87,13 +88,13 @@ pub struct OrderCancelledEvent {
 }
 
 /// Provides a way to get the id of the Order events
-impl OrderEvent {
+impl Identifier for OrderEvent {
     #[allow(dead_code)]
-    pub fn id(&self) -> u32 {
+    fn identifier(&self) -> String {
         match self {
-            OrderEvent::Created(c) => c.order_id.to_owned(),
-            OrderEvent::Updated(c) => c.order_id.to_owned(),
-            OrderEvent::Cancelled(c) => c.order_id.to_owned(),
+            OrderEvent::Created(c) => c.order_id.to_string(),
+            OrderEvent::Updated(c) => c.order_id.to_string(),
+            OrderEvent::Cancelled(c) => c.order_id.to_string(),
         }
     }
 }
@@ -136,11 +137,11 @@ pub struct CreateShipmentCommand {
 }
 
 /// Provides a way to get the id of the Shipment commands
-impl ShipmentCommand {
+impl Identifier for ShipmentCommand {
     #[allow(dead_code)]
-    pub fn id(&self) -> u32 {
+    fn identifier(&self) -> String {
         match self {
-            ShipmentCommand::Create(c) => c.shipment_id.to_owned(),
+            ShipmentCommand::Create(c) => c.shipment_id.to_string(),
         }
     }
 }
diff --git a/tests/application/mod.rs b/tests/application/mod.rs
index 3eaa7fd..3eaf68d 100644
--- a/tests/application/mod.rs
+++ b/tests/application/mod.rs
@@ -1,12 +1,11 @@
 use derive_more::Display;
-use fmodel_rust::Sum;
+use fmodel_rust::{Identifier, Sum};
 use std::error::Error;
 
 use crate::api::{
     CancelOrderCommand, CreateOrderCommand, CreateShipmentCommand, OrderCancelledEvent,
-    OrderCommand, OrderCreatedEvent, OrderEvent, OrderState, OrderUpdatedEvent, OrderViewState,
-    ShipmentCommand, ShipmentCreatedEvent, ShipmentEvent, ShipmentState, ShipmentViewState,
-    UpdateOrderCommand,
+    OrderCommand, OrderCreatedEvent, OrderEvent, OrderUpdatedEvent, ShipmentCommand,
+    ShipmentCreatedEvent, ShipmentEvent, UpdateOrderCommand,
 };
 
 /// The command enum for all the domain commands (shipment and order)
@@ -105,46 +104,28 @@ pub fn sum_to_event(event: &Sum<OrderEvent, ShipmentEvent>) -> Event {
     }
 }
 
-/// A trait to provide a way to get the id of the messages/entities
-#[allow(dead_code)]
-pub trait Id {
-    fn id(&self) -> u32;
-}
-
-impl Id for Event {
-    fn id(&self) -> u32 {
+impl Identifier for Event {
+    fn identifier(&self) -> String {
         match self {
-            Event::OrderCreated(event) => event.order_id,
-            Event::OrderCancelled(event) => event.order_id,
-            Event::OrderUpdated(event) => event.order_id,
-            Event::ShipmentCreated(event) => event.shipment_id,
+            Event::ShipmentCreated(evt) => evt.shipment_id.to_string(),
+            Event::OrderCreated(evt) => evt.order_id.to_string(),
+            Event::OrderUpdated(evt) => evt.order_id.to_string(),
+            Event::OrderCancelled(evt) => evt.order_id.to_string(),
         }
     }
 }
 
-impl Id for Command {
-    fn id(&self) -> u32 {
+impl Identifier for Command {
+    fn identifier(&self) -> String {
         match self {
-            Command::OrderCreate(cmd) => cmd.order_id,
-            Command::OrderUpdate(cmd) => cmd.order_id,
-            Command::OrderCancel(cmd) => cmd.order_id,
-            Command::ShipmentCreate(cmd) => cmd.shipment_id,
+            Command::OrderCreate(cmd) => cmd.order_id.to_string(),
+            Command::OrderUpdate(cmd) => cmd.order_id.to_string(),
+            Command::OrderCancel(cmd) => cmd.order_id.to_string(),
+            Command::ShipmentCreate(cmd) => cmd.shipment_id.to_string(),
         }
     }
 }
 
-impl Id for (OrderState, ShipmentState) {
-    fn id(&self) -> u32 {
-        self.0.order_id
-    }
-}
-
-impl Id for (OrderViewState, ShipmentViewState) {
-    fn id(&self) -> u32 {
-        self.0.order_id
-    }
-}
-
 /// Error type for the application/aggregate
 #[derive(Debug, Display)]
 #[allow(dead_code)]
diff --git a/tests/materialized_view_combined_test.rs b/tests/materialized_view_combined_test.rs
index 8e5eff0..fe80960 100644
--- a/tests/materialized_view_combined_test.rs
+++ b/tests/materialized_view_combined_test.rs
@@ -4,12 +4,13 @@ use std::thread;
 
 use fmodel_rust::materialized_view::{MaterializedView, ViewStateRepository};
 use fmodel_rust::view::View;
+use fmodel_rust::Identifier;
 
 use crate::api::{
     OrderCancelledEvent, OrderCreatedEvent, OrderEvent, OrderUpdatedEvent, OrderViewState,
     ShipmentEvent, ShipmentViewState,
 };
-use crate::application::{event_from_sum, Event, Id, MaterializedViewError};
+use crate::application::{event_from_sum, Event, MaterializedViewError};
 
 mod api;
 mod application;
@@ -85,7 +86,12 @@ impl ViewStateRepository<Event, (OrderViewState, ShipmentViewState), Materialize
         &self,
         event: &Event,
     ) -> Result<Option<(OrderViewState, ShipmentViewState)>, MaterializedViewError> {
-        Ok(self.states.lock().unwrap().get(&event.id()).cloned())
+        Ok(self
+            .states
+            .lock()
+            .unwrap()
+            .get(&event.identifier().parse::<u32>().unwrap())
+            .cloned())
     }
 
     async fn save(
@@ -95,7 +101,7 @@ impl ViewStateRepository<Event, (OrderViewState, ShipmentViewState), Materialize
         self.states
             .lock()
             .unwrap()
-            .insert(state.id(), state.clone());
+            .insert(state.0.order_id, state.clone());
         Ok(state.clone())
     }
 }
diff --git a/tests/materialized_view_test.rs b/tests/materialized_view_test.rs
index 5c7325f..3bf401d 100644
--- a/tests/materialized_view_test.rs
+++ b/tests/materialized_view_test.rs
@@ -4,6 +4,7 @@ use std::thread;
 
 use fmodel_rust::materialized_view::{MaterializedView, ViewStateRepository};
 use fmodel_rust::view::View;
+use fmodel_rust::Identifier;
 
 use crate::api::{
     OrderCancelledEvent, OrderCreatedEvent, OrderEvent, OrderUpdatedEvent, OrderViewState,
@@ -61,7 +62,12 @@ impl ViewStateRepository<OrderEvent, OrderViewState, MaterializedViewError>
         &self,
         event: &OrderEvent,
     ) -> Result<Option<OrderViewState>, MaterializedViewError> {
-        Ok(self.states.lock().unwrap().get(&event.id()).cloned())
+        Ok(self
+            .states
+            .lock()
+            .unwrap()
+            .get(&event.identifier().parse::<u32>().unwrap())
+            .cloned())
     }
 
     async fn save(&self, state: &OrderViewState) -> Result<OrderViewState, MaterializedViewError> {