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Rules

Rules Summary

The table shows excerpts only; see the nw sample for full syntax.

Rule Summary Example Notes
Constraint Boolean function must be True
else transaction rolled back		 row.Balance <= row.CreditLimit
row.Salary >= Decimal('1.20') * old_row.Salary		 Multi-field
old_row
Formula Function computes column value row.UnitPrice * row.Quantity
row.OrderHeader.ShippedDate		 lambda, or function
Parent (OrderHeader) references
Sum Derive parent-attribute as sum of designated child attribute; optional child qualification Rule.sum(derive=Customer.Balance, as_sum_of=Order.AmountTotal,where=lambda row: row.ShippedDate is None) Parent attribute can be hybrid (virtual)
scalable: pruning, adjustment
Count Derive parent-attribute as count of child rows; optional child qualification Rule.sum(derive=Customer.Balance, as_sum_of=Order.AmountTotal,where=lambda row: row.ShippedDate is None) Parent attribute can be hybrid (virtual)
scalable: pruning, adjustment
Copy Child value set from Parent OrderDetail.ProductPrice = copy(Product.Price) Unlike formula references, parent changes are not propagated
e.g, Order totals for Monday are not affected by a Tuesday price increase
Event Python Function on insert, call congratulate_sales_rep See Extensibility for a information on early, row and commit events
Parent Check Ensure Parent row exists Orders must have a Customer See Referential Integrity
Allocation Allocate a provider amount to recipients allocate a payment to outstanding orders See Allocation for an example
Copy Row Create child row by copying parent audit Employee Salary changes to EmployeeAudit See Rule Extensibility

 

Learning Rules

Inside the larger process above, here is the best way to learn how to use rules:

  1. Rule Summary: there are a small number of rules, since their power lies in chaining (duplicated below)

  2. Be aware of the rule patterns, duplicated below

  3. Use the case study approach to learn about using rules, by exploring the examples in the report, below.

  4. Be aware of Rule Extensibility.

  

Case Study

The best way to learn the rules is by a Case Study approach:

  1. See the Behave Logic Report

  2. On each, open the disclosure box: "Tests - and their logic - are transparent.. click to see Logic"

Rule Patterns

Pattern Notes Example
Chain Up parent sums and counts mean that child row changes can adjust parents Derive Balance
Chain Down child copy and parent references mean that parent row changes can cascade to children Ship Order
Constrain a Derived Result constraints may require derived values Balance < creditLimit
Auditing Note the Copy Row rule Salary Audit
old_row useful for state transition logic Meaningful Raise