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Logic: Rule Types, Patterns

💡 TL;DR - Logic: multi-table derivations and constraints, using Rules and Python

Logic addresses multi-table derivations and constraints, using Rules and Python.

Rules are:

  1. Declared in your IDE - 40X more concise - with GenAI or Code Completion
  2. Activated on server start
  3. Executed - automatically - on updates (using SQLAlchemy events)
  4. Debugged in your IDE, and with the console log

Rule Types

The table shows excerpts only; see the ApiLogicProject (Northwind) 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
Formula Function computes column value row.UnitPrice * row.Quantity
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=Order.OrderDetailCount, as_count_of=OrderDetail) counts are useful as child 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


Declaring Rules

Rules are declared in your IDE, using GenAI or Code Completion. Edit the file in your project, or, one of the files in the discovery directory.


With GenAI Logic Automation

If you have Copilot installed, you can use GenAI to create rules. Enter your GenAI prompt, and use Copilot's Insert at Cursor button to pasted into

You will typically need to make some small changes, such as the import statements.

In the example below, we have pasted the prompt from the GenAI Automation prompt used to create the system. You can of course enter new text, or paste from any source.

Add Rules GenAI


With Code Completion

You can also use Code Completion to add rules, and their arguments.

Add Rules Code Completion


Iterative Rules

Logic definition is an incremental process. You can start with a few rules, and add more as needed. There is no need to define all rules at once, or rebuild the project.

Note rules are automatically ordered and invoked, so you can add new ones in any location.

Similarly, you can change rules without worrying about the order of execution.


Learning Rules

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

  1. Rule Summary: review the table above; there are a small number of rules, since their power lies in chaining

    • Alert: Logic consists of rules and Python. You will quickly learn to use logic events; focus on the rules as the preferred approach, using Python (events, etc) as a fallback.
  2. Review the Rule Patterns, 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.

Pre-req: before learning rules, use the Tutorial to familiarize yourself with basic capabilities and procedures.


Rule Patterns

Pattern Notes Example
Chain Up parent sums and counts mean that child row changes can adjust parents Derive Balance
Constrain a Derived Result constraints may require derived values Balance < creditLimit
Chain Down child copy and parent references mean that parent row changes can cascade to children Ship Order
State Transition Logic old_row useful comparing old/current values Meaningful Raise
Counts as Existence Checks Use counts to check if any children exist Don't Ship Empty Orders
Auditing Note the Copy Row rule (and alternatives) Salary Audit
Ready Flag Multi-session editing, then , when ready...
adjust related data / enforce constraints
Ship Order,
Make Order Ready
Events for Lib Access Events enable Python, use of standard libs (e.g., Kafka) Ship Order


Case Study

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

  1. Print this page, for reference

  2. Print the Database Diagram

    • Most of the examples are drawn from this database
  3. For each Rule Pattern, above:

    • Click the Example link in the table above to open the Behave Logic Report

      • Aside: later, you can prepare such documentation for your own projects, (like this).
    • Review the Scenario -- take these as your requirements

    • Spend 5 minutes (perhaps in pairs) and cocktail-napkin design your solution, using

      • The data model diagram
      • List of Rule Types, and
      • Rule Patterns
    • Reveal the solution: open the disclosure box: "Tests - and their logic - are transparent.. click to see Logic"