diff --git a/be/app/core/exceptions.py b/be/app/core/exceptions.py
index 39479cf..aedd157 100644
--- a/be/app/core/exceptions.py
+++ b/be/app/core/exceptions.py
@@ -354,8 +354,4 @@ class PermissionDeniedError(HTTPException):
             detail=detail
         )
 
-# Financials & Cost Splitting specific errors
-class BalanceCalculationError(HTTPException):
-    # This class is not provided in the original file or the code block
-    # It's assumed to exist as it's called in the code block
-    pass 
\ No newline at end of file
+# Financials & Cost Splitting specific errors
\ No newline at end of file
diff --git a/be/app/models.py b/be/app/models.py
index 74fbba1..cc4d10f 100644
--- a/be/app/models.py
+++ b/be/app/models.py
@@ -251,12 +251,16 @@ class Expense(Base):
     group = relationship("Group", foreign_keys=[group_id], back_populates="expenses")
     item = relationship("Item", foreign_keys=[item_id], back_populates="expenses")
     splits = relationship("ExpenseSplit", back_populates="expense", cascade="all, delete-orphan")
+    parent_expense = relationship("Expense", remote_side=[id], back_populates="child_expenses")
+    child_expenses = relationship("Expense", back_populates="parent_expense")
     overall_settlement_status = Column(SAEnum(ExpenseOverallStatusEnum, name="expenseoverallstatusenum", create_type=True), nullable=False, server_default=ExpenseOverallStatusEnum.unpaid.value, default=ExpenseOverallStatusEnum.unpaid)
     # --- Recurrence fields ---
     is_recurring = Column(Boolean, default=False, nullable=False)
     recurrence_pattern_id = Column(Integer, ForeignKey("recurrence_patterns.id"), nullable=True)
     recurrence_pattern = relationship("RecurrencePattern", back_populates="expenses", uselist=False) # One-to-one
     next_occurrence = Column(DateTime(timezone=True), nullable=True) # For recurring expenses
+    parent_expense_id = Column(Integer, ForeignKey("expenses.id"), nullable=True)
+    last_occurrence = Column(DateTime(timezone=True), nullable=True)
 
     __table_args__ = (
         # Ensure at least one context is provided