Aggregator

The Aggregator primitive computes a value over child records of a parent and writes the result to a parent field. It supports five reducers (sum, count, avg, min, max) and an optional where: filter so you can compute aggregates over a subset of children.

Configuration

Field	Type	Description
`source_entity`	string	The child entity name (e.g. `OrderLine`).
`parent_entity`	string	The parent entity name (e.g. `Order`).
`parent_link_field`	string	The field on the child that holds the parent's id (e.g. `order_id`).
`aggregate`	enum	One of `sum`, `count`, `avg`, `min`, `max`.
`source_field`	string	The child field to aggregate. Required unless `aggregate=count`.
`target_field`	string	The parent field that receives the result.
`where`	JSONLogic object	Optional. Only child rows where the predicate evaluates true are included.

Example: Sum of approved-only line items

[[bindings]]
primitive = "fastyoke.aggregator"
trigger = { kind = "on_field_change", entity = "OrderLine", field = "amount" }
config = {
  source_entity = "OrderLine",
  parent_entity = "Order",
  parent_link_field = "order_id",
  aggregate = "sum",
  source_field = "amount",
  target_field = "approved_total",
  where = { "==": [{ var = "status" }, "approved"] }
}

When any OrderLine.amount changes, Aggregator recomputes the sum of amount across all OrderLine rows linked to the same Order where status == "approved", and writes the result to Order.approved_total.

When to use it

Order totals from line items.
Payment running totals from a payment ledger (combined with where: filter on status).
Inventory on-hand summed across all warehouses for a single SKU.

What it can't do

Atomic delta-write — Aggregator recomputes from children. For "add 5 to this counter" use Field Mutator.
Cross-entity lookup — Aggregator works on a single parent-child relationship. To denormalize a value from a different entity, see Field Compute.