diff --git a/01.workspace/heave/src/fun/sqlite_init_db.rs b/01.workspace/heave/src/fun/sqlite_init_db.rs
index 224e5e3..9deb235 100644
--- a/01.workspace/heave/src/fun/sqlite_init_db.rs
+++ b/01.workspace/heave/src/fun/sqlite_init_db.rs
@@ -5,7 +5,8 @@ pub fn run(path: &path::Path) {
     let init_statement = r#"
         CREATE TABLE IF NOT EXISTS entity (
             id TEXT PRIMARY KEY,
-            class TEXT NOT NULL
+            class TEXT NOT NULL,
+            ref_date INTEGER NOT NULL
         );
         CREATE TABLE IF NOT EXISTS attribute (
             id TEXT,
diff --git a/01.workspace/heave/src/fun/sqlite_map_row_to_entity.rs b/01.workspace/heave/src/fun/sqlite_map_row_to_entity.rs
index a0512e4..53daac1 100644
--- a/01.workspace/heave/src/fun/sqlite_map_row_to_entity.rs
+++ b/01.workspace/heave/src/fun/sqlite_map_row_to_entity.rs
@@ -3,7 +3,11 @@ use crate::*;
 pub fn run(row: &rusqlite::Row) -> rusqlite::Result<Entity> {
     let id: String = row.get(0)?;
     let class: String = row.get(1)?;
-    Ok(Entity::default().with_id(&id).with_class(&class))
+    let ref_date: u64 = row.get(2)?;
+    Ok(Entity::default()
+        .with_id(&id)
+        .with_class(&class)
+        .with_ref_date(ref_date))
 }
 
 // #[cfg(test)]
diff --git a/01.workspace/heave/src/fun/sqlite_persist_catalog.rs b/01.workspace/heave/src/fun/sqlite_persist_catalog.rs
index 7ead46a..b38b0bc 100644
--- a/01.workspace/heave/src/fun/sqlite_persist_catalog.rs
+++ b/01.workspace/heave/src/fun/sqlite_persist_catalog.rs
@@ -17,8 +17,8 @@ const DELETE_ENTITY_STATEMENT: &str = r#"
 "#;
 
 const INSERT_ENTITY_STATEMENT: &str = r#"
-    INSERT INTO entity (id, class)
-    VALUES (?1, ?2);
+    INSERT INTO entity (id, class, ref_date)
+    VALUES (?1, ?2, ?3);
 "#;
 
 const INSERT_ATTRIBUTE_STATEMENT_TEMPLATE: &str = r#"
@@ -36,7 +36,7 @@ fn write_attribute(attribute: &Attribute, entity: &Entity, transaction: &rusqlit
 
 fn write_entity(entity: &Entity, transaction: &rusqlite::Transaction) {
     let entity_id = [&entity.id];
-    let entity_values = (&entity.id, &entity.class);
+    let entity_values = (&entity.id, &entity.class, entity.ref_date);
     let _ = transaction.execute(DELETE_ENTITY_STATEMENT, entity_id);
     let _ = transaction.execute(INSERT_ENTITY_STATEMENT, entity_values);
     for (_key, attribute) in entity.attributes.iter() {
diff --git a/01.workspace/heave/src/str/entity.rs b/01.workspace/heave/src/str/entity.rs
index e080a3c..61cfa9f 100644
--- a/01.workspace/heave/src/str/entity.rs
+++ b/01.workspace/heave/src/str/entity.rs
@@ -4,6 +4,7 @@ use crate::*;
 pub struct O {
     pub id: String,
     pub state: EntityState,
+    pub ref_date: u64,
     pub class: String,
     pub attributes: std::collections::HashMap<String, Attribute>,
 }
@@ -26,6 +27,10 @@ impl O {
         self.class = class.to_string();
         self
     }
+    pub fn with_ref_date(mut self, ref_date: u64) -> Self {
+        self.ref_date = ref_date;
+        self
+    }
     pub fn with_attribute(mut self, id: &str, value: impl Into<Value>) -> Self {
         let attribute = Attribute::new(id, value);
         self.attributes.insert(attribute.id.clone(), attribute);