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💡 TL;DR - Derive attribute using other attributes of current/parent class

Formula rules are lambda expressions or functions that can reference other attributes of current/parent class. They are declared in the declare_logic() function in logic/

Execution order is system-determined based on dependencies.

Changes to referenced parent attributes are propagated to all child rows. Contrast this to the Copy rule, for parent references where you do not want propagation.

Formulas may be pruned if there are no changes to the referenced attributes.


Provide expression as lamda

Simple formulas are most easily expressed as lambda functions:

Rule.formula(derive=models.OrderDetail.Amount,  # compute price * qty
        as_expression=lambda row: row.UnitPrice * row.Quantity)


Provide expression as function

   def derive_amount(row: models.Item, old_row: models.Item, logic_row: LogicRow):
       amount = row.Quantity * row.UnitPrice
       if row.Product.CarbonNeutral and row.Quantity >= 10:
           amount = amount * Decimal(0.9)
       return amount

   Rule.formula(derive=models.Item.Amount, calling=derive_amount)