Examples

As shown in the Rules Summary, the list of rules is small. You can learn them in under an hour.

Rules are the "words", and, like writing, their power comes from using them in combination.. This page summarizes the rules, and illustrates combining them to solve several non-trivial examples.

   

Rules Summary

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

https://github.com/valhuber/LogicBank/wiki/Copy-Children

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.count(derive=Customer.OrderCount, as_count_of=Order, where=lambda row: row.ShippedDate is None)	Useful for existence checks
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
Copy Children	Create multiple types of child rows by copying designated children (deep copy)	Clone Order - create a new order, populating OrderDetails from existing order	See Rule-Extensibility See Clone Order

   

Learning Rules

The best way to learn Logic Bank:

1. Review the list of rules above

2. Then, review the examples below:

Sample	Requirement	Illustrates Patterns
Check Credit Rules are 40X more concise than legacy code	Compute OrderDetail.Amount, sum OrderDetails into Order & Customer, verify Balance <= Limit	Constrain a derived result Chaining Qualified Sum
Sufficient Raise	Raises must exceed 20%	Use of old_row
Ship Order	When Orders are shipped, reduce the Customer.Balance, and Product.UnitsInStock	Illustrates cascade, another form of multi-table logic
Orders for Commissioned Employees Only	Ensure that the Order.EmployeeId references an Employee that IsCommissioned	Counts as child existence checks
Allocation	Allocate Payment to a set of outstanding Orders, reducing CustomerBalance	illustrates extensibility, providing a reusable pattern: A provider allocates to a set of recipients, plus rollups
Banking	Funds Transfer between accounts	A complex transaction using the command pattern

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Suggestion: try to imagine the solution, then read the article.