README.rst

Python Rules - Logic for SQLAlchemy

This package enables you to declare rules that govern SQLAlchemy update transaction logic (multi-table derivations, constraints, and actions such as sending mail or messages).

Logic is stated in Python, and is over an 40X more concise than code.

Features

Logic is declared in Python (example below), and is:

• Multi-table: rules like sum automate multi-table transactions
• Scalable: rules are pruned and optimized; for example, sums are processed as 1 row adjustment updates, rather than expensive SQL aggregate queries
• Extensible: use Python to implement any logic not addressed by rules
• Manageable: develop and debug your rules in IDEs, manage it in SCS systems (such as git) using existing procedures

Example:

The following 5 rules represent the same logic as 200 lines of Python:

def activate_basic_check_credit_rules():
    """ Check Credit Requirement:
        * the balance must not exceed the credit limit,
        * where the balance is the sum of the unshipped order totals
        * which is the rollup of OrderDetail Price * Quantities:
    """

    Rule.constraint(validate=Customer, as_condition=lambda row: row.Balance <= row.CreditLimit,
                    error_msg="balance ({row.Balance}) exceeds credit ({row.CreditLimit})")
    Rule.sum(derive=Customer.Balance, as_sum_of=Order.AmountTotal,
             where=lambda row: row.ShippedDate is None)  # *not* a sql select sum

    Rule.sum(derive=Order.AmountTotal, as_sum_of=OrderDetail.Amount)

    Rule.formula(derive=OrderDetail.Amount, as_expression=lambda row: row.UnitPrice * row.Quantity)
    Rule.copy(derive=OrderDetail.UnitPrice, from_parent=Product.UnitPrice)

Depends on:

• SQLAlchemy
• Python 3.8

More information:

See the python-rules github for more information, and explore the code.

Acknowledgements

Many thanks to

• Tyler Band, for early testing

Change Log

0.0.2 - fix imports