Sum

TL;DR - Derive parent-attribute as sum of designated child attribute; optional child qualification

Sum derivations enable you to aggregate child data. They are declared in the declare_logic() function in logic/declare_logic.py.

Sums are efficient: child changes result in a 1 row adjustment update, not an expensive select sum. And, they are pruned entirely if child does not alter the summed field or the qualification condition.

 

Defining Sums

You declare sums in xxx:

Rule.sum(derive=models.Customer.Balance,        # adjust iff AmountTotal or ShippedDate or CustomerID changes
        as_sum_of=models.Order.AmountTotal,
        where=lambda row: row.ShippedDate is None)  # adjusts - *not* a sql select sum...

Insert Parent option

In most cases, the parent must exist or an exception will be thrown. However, you may wish to create "group by" aggregates rows; see the Budget App.

You can achieve this effect with the insert_parent parameter:

Rule.sum(derive=models.YrTotal.budget_total, as_sum_of=models.CategoryTotal.budget_total,insert_parent=use_parent_insert)

Multiple Relationships to the Same Parent

If the child and parent classes are connected by more than one relationship - for example, an Employee that can be works_for one Department and on_loan to another - Rule.sum cannot infer which relationship to follow, and raises Ambiguous Relationship.

Resolve this with child_role_name, naming the relationship attribute declared on the child class:

Rule.sum(derive=models.Department.works_for_salary_total, as_sum_of=models.Employee.salary,
         child_role_name="works_for_dept")
Rule.sum(derive=models.Department.on_loan_salary_total, as_sum_of=models.Employee.salary,
         child_role_name="on_loan_dept")

child_role_name is only needed when ambiguity exists - the typical single-relationship case omits it entirely. The same parameter is available on Rule.count and Rule.copy for the same reason.