# One store per action

An action writes to at most one [store](stores.md), 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`](../errors/store.action_spans_stores.md)
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:

```jadpo
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
        }
    }
}
```

```text
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

check failed: 1 error, 5 files
```