# Writing to Several Subjects

A command acts on a single subject, since it is a decision on the state of that subject. Some events are no such decision, for example a summary derived from many subjects, which has to be written together with a note on each of them, either all or none. To write events to several subjects at once, call the `Write` function with a context, the store, the events together with their subjects, and the preconditions:

```go
written, err := architecturekit.Write(context.TODO(), store, []architecturekit.EventOn{
  {Subject: "/inventories/2026", Event: InventoryTaken{BookCount: 2}},
  {Subject: "/books/42", Event: BookCounted{Inventory: "2026"}},
  {Subject: "/books/23", Event: BookCounted{Inventory: "2026"}},
}, architecturekit.Require(eventsourcingdb.NewIsSubjectPristinePrecondition("/inventories/2026")))
if errors.Is(err, architecturekit.ErrConflict) {
  // Somebody else has taken the inventory already.
}
```

`Write` writes the events with the source of the store, and returns them as the database recorded them. If a precondition does not hold, nothing is written, and the error belongs to the category `ErrConflict`. Other failures belong to the same categories as for `Execute` (see [Handling Errors](/docs/architecturekit/handling-errors)).

Like a command, a write declares at least one precondition, made with `Require`, or `Unconditionally` to write without any. `OnStateRead` has nothing to guard, since `Write` reads no state.

*Note that `Write` never decides again, since there is nothing to decide. If nothing is to be written, call it with no events, which writes nothing.*