# Testing Projections

To test a projection without a database, call the `Project` function with a `*testing.T`, the projection, and the stored events. To turn typed events for the same subject into stored ones, call the `StoredEvents` function, which numbers them from `0`:

```go
func TestCatalogProjection(t *testing.T) {
  catalog := architecturekit.NewItemView[BookItem]()
  catalogProjection := CatalogProjection{catalog: catalog}

  architecturekittest.Project(t, catalogProjection,
    architecturekittest.StoredEvents("/books/42",
      BookAcquired{
        Title:  "2001 – A Space Odyssey",
        Author: "Arthur C. Clarke",
        ISBN:   "978-0756906788",
      },
      BookBorrowed{
        BorrowedBy:    "23",
        BorrowedUntil: "2026-10-24",
      },
    )...,
  )

  architecturekittest.ExpectItems(t, catalog, BookItem{
    ID:            "42",
    Title:         "2001 – A Space Odyssey",
    Author:        "Arthur C. Clarke",
    IsBorrowed:    true,
    BorrowedUntil: "2026-10-24",
    EventID:       "1",
  })
}
```

The `ExpectItems` function expects the view to hold exactly the given items, in the given order. It requires an item type that is comparable. To get the items as a slice instead, call the `ItemsOf` function:

```go
items := architecturekittest.ItemsOf(t, catalog)
```

To check how a projection will be run, call the `ExpectMode` function:

```go
architecturekittest.ExpectMode(t, catalogProjection, architecturekit.ModeRebuild)
```

To test a transactional projection, call the `ProjectTransactional` function instead of `Project`. It begins a transaction, applies the events, and commits the transaction with the ID of the last event. If an event is refused, it rolls the transaction back:

```go
architecturekittest.ProjectTransactional(t, &TransactionalBookTableProjection{},
  architecturekittest.StoredEvents("/books/42",
    BookAcquired{
      Title:  "2001 – A Space Odyssey",
      Author: "Arthur C. Clarke",
      ISBN:   "978-0756906788",
    },
  )...,
)
```

*Note that `Project` fails the test for a projection that implements `Transactional` in addition to `Apply`.*