Copy

TL;DR - Derive Attribute from parent -- parent changes do not propagate

Copy rules provide parent access, without propagation. They are declared in the declare_logic() function in logic/declare_logic.py.

Constrast this to Formula derivation rules which enable parent references, with propogation.

 

Declare Copy Rule

In this example, Product UnitPrice changes are not propagated to existing OrderDetails. So, your Monday purchase is not affected by a Tuesday price change:

Rule.copy(derive=models.OrderDetail.UnitPrice,  # get Product Price (e,g., on insert, or ProductId change)
        from_parent=models.Product.UnitPrice)