One store per action

An action writes to at most one store, counting the public actions it calls. It may read other stores, but those reads are outside its transaction. A read that decides a write (a policy, lifecycle or invariant check) must come from the store being written.

Across stores there are no joins, filters or ordering. An include reaches another store only to fetch records by id.

Moving an entity to another store can break an action that never names it, so the error names the cause: store.action_spans_stores points at the write or the call that brings in the second store.

At run time, an action's transaction is begun in the store its first store operation names. Reading another store outside the transaction isn't built yet. For now, an action that reads a second store, itself or through an action it calls, is toolchain.not_built_yet at the read or call that brings it in, rather than failing as it runs:

example

use ordering/entities { Order, Product, ProductNotFound }action order_product(product_id: Product.id) fails ProductNotFound -> Order {    var product: Product = attempt Product.get(product_id)    return Order.create {        set: {            product_name: product.name        }    }}

jadpo checkcheck failed: 1 error, 5 files

error Reading a second store in order_product isn't built yet

--> domains/ordering/actions/order_product.jadpo:4:36

  |4 |     var product: Product = attempt Product.get(product_id)  |                                    ^^^^^^^^^^^^^^^^^^^^^^^ store `catalogue`

reason: An action may read another store outside its transaction (area 6), but this compiler runs all of an action's store operations in the one transaction its first begins; order_product uses Order (store main), and here it reads Product (store catalogue).
next: Read Product before order_product runs, and pass in what it needs, for now (agent)
rule: toolchain.not_built_yet
help: https://jadpo.dev/docs/errors/toolchain.not_built_yet

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