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Using Logic

💡 TL;DR - Using Logic: Understand functions vs. lambda, logic_row

For constraints / formulas that require an expression, you can use lambdas or functions.

Functions have a logic_row arguments, providing access to old/new row, verb, etc.


Declaration and Activation

Declared rules are automatically activated by when it starts the server. For more information, please see Project Operation.


Function vs. Lambda

Constraint and Formula rules have arguments for providing an expression. There are two ways to do this.

For simple expressions, provide a lambda (inline) function, e.g.:

    Rule.formula(derive=models.OrderDetail.Amount,  # compute price * qty
        as_expression=lambda row: row.UnitPrice * row.Quantity)

To get access to old_row and logic_row, specify a function, e.g.:

        Demonstrate that logic == Rules + Python (for extensibility)
    def congratulate_sales_rep(row: models.Order, old_row: models.Order, logic_row: LogicRow):
        """ use events for sending email, messages, etc. """
        if logic_row.ins_upd_dlt == "ins":  # logic engine fills parents for insert
            sales_rep = row.Employee        # parent accessor
            if sales_rep is None:
                logic_row.log("no salesrep for this order")
            elif sales_rep.Manager is None:
                logic_row.log("no manager for this order's salesrep")
                logic_row.log(f'Hi, {sales_rep.Manager.FirstName} - '
                              f'Congratulate {sales_rep.FirstName} on their new order')
            category_1 = logic_row.session.query(models.Category).filter(models.Category.Id == 1).one()
            logic_row.log("Illustrate database access (not subject to authorization)")  # not granted for user: u2

    Rule.commit_row_event(on_class=models.Order, calling=congratulate_sales_rep)


LogicRow: old_row, verb, etc

From the online documentation (use code completion):

💡 TL;DR - LogicRow provides access to old_row, ins_upd_dlt etc.

Wraps row and old_row, plus methods for insert, update and delete - rule enforcement

Additional instance variables: ins_upd_dlt, nest_level, session, etc.

Helper Methods

    are_attributes_changed, set_same_named_attributes, get_parent_logic_row(role_name), get_derived_attributes, log, etc

Here is an example:

    STATE TRANSITION LOGIC, using old_row
def raise_over_20_percent(row: models.Employee, old_row: models.Employee, logic_row: LogicRow):
    if logic_row.ins_upd_dlt == "upd" and row.Salary > old_row.Salary:
        return row.Salary >= Decimal('1.20') * old_row.Salary
        return True
                error_msg="{row.LastName} needs a more meaningful raise")

Note the log method, which enables you to write row/old_row into the log with a short message:

logic_row.log("no manager for this order's salesrep")

Use code completion to see the full list of methods and properties:




Inserting coding depends on whether you are already in the context of a logic_row ("in logic"), or not ("in APIs and Messages"). These are described below.


In APIs and Messages

In API/Message development, you can submit transaction payloads as shown below:

class ServicesEndPoint(safrs.JABase):

    def OrderB2B(self, *args, **kwargs):  # yaml comment => swagger description
        """ # yaml creates Swagger description
            args :
                    AccountId: "ALFKI"
                    Given: "Steven"
                    Surname: "Buchanan"
                    Items :
                    - ProductName: "Chai"
                      QuantityOrdered: 1
                    - ProductName: "Chang"
                      QuantityOrdered: 2

        Note attribute alias, Lookup automation in OrderB2B

        Test with swagger, or, from command line:

        $(venv) ApiLogicServer login --user=admin --password=p
        $(venv) ApiLogicServer curl "'POST' 'http://localhost:5656/api/ServicesEndPoint/OrderB2B'" --data '
        {"meta": {"args": {"order": {
            "AccountId": "ALFKI",
            "Surname": "Buchanan",
            "Given": "Steven",
            "Items": [
                "ProductName": "Chai",
                "QuantityOrdered": 1
                "ProductName": "Chang",
                "QuantityOrdered": 2


        db = safrs.DB         # Use the safrs.DB, not db!
        session = db.session  # sqlalchemy.orm.scoping.scoped_session

        order_b2b_def = OrderB2B()
        request_dict_data = request.json["meta"]["args"]["order"]
        sql_alchemy_row = order_b2b_def.dict_to_row(row_dict = request_dict_data, session = session)

        return {"Thankyou For Your OrderB2B"}  # automatic commit, which executes transaction logic

Salient points:

  1. Note the process of obtaining a session from safrs (the api engine)
  2. Note the use of the OrderB2B "RowDictMapper" to transform payload data into row instances
  3. Observe that you add the row to the session; logic execution is automatic


In Logic

In logic development, before you save a new row, you will first need to instantiate a new row instance. Consider the following code from the sample app logic/

    if preferred_approach:  # #als: AUDITING can be as simple as 1 rule
                            copy_when=lambda logic_row: logic_row.ins_upd_dlt == "upd" and 
                                    logic_row.are_attributes_changed([models.Employee.Salary, models.Employee.Title]))
        def audit_by_event(row: models.Employee, old_row: models.Employee, logic_row: LogicRow):
            tedious = False  # tedious code to repeat for every audited class
            if tedious:      # see instead the RuleExtension.copy_row above (you can create similar rule extensions)
                if logic_row.ins_upd_dlt == "upd" and logic_row.are_attributes_changed([models.Employee.Salary, models.Employee.Title]):
                    # #als: triggered inserts  
                    copy_to_logic_row = logic_row.new_logic_row(models.EmployeeAudit)
                    copy_to_logic_row.insert(reason="Manual Copy " +  # triggers rules...

        Rule.commit_row_event(on_class=models.Employee, calling=audit_by_event)

If you are doing auditing (a common pattern), we recommend you consider copy_row. The alternative coding illustrates how do do insert manually, using logic_row.new_logic_row(models.EmployeeAudit).


Insert defaulting

In either case, the new logic row is returned with default values. These are obtained from your database schema, via database/ For example:


Observe the server_default property. This value is used by LogicBank, as follows:

  1. Defaults are applied when instantiating a new row, and for default values when saving an inserted row

  2. Only constant values are defaulting, and dates and datetimes with the value CURRENT_TIMESTAMP

  3. Defaulted values are depicted on the server console


Updating and Deleting

These require you first obtain the row, either through a model class accessor (e.g., retrieve a parent row), or a SQLAlchemy call. In any case, alter the row as required, and issue `logic_row.update()' (or delete). As for insert, this triggers logic execution.