# `Chronicle.Testing.Projections`
[🔗](https://github.com/Cratis/Chronicle.Elixir/blob/main/lib/chronicle/testing/projections.ex#L4)

Evaluates the supported subset of a `Chronicle.ReadModels.ReadModel`'s embedded
projection declarations against events, in memory — for a spec that wants to
assert on a projected read model without a kernel to run the real projection
engine.

Supports `from/2` (`:set`, `:add`, `:subtract`, `:count`, `:increment`,
`:decrement`, `:clear`) and auto-map, plus `removed_with/2`.

`join/2`, `children/2`, `from_every/1`, `removed_with_join/2` and their
variants need the server-side projection engine and raise
`Chronicle.Testing.UnsupportedProjectionOperation` — point a spec at a real
kernel through `Chronicle.Testing.Integration.setup_isolated_client/2` instead.

# `apply_event`

```elixir
@spec apply_event(module(), struct() | nil, struct(), String.t()) ::
  {:ok, struct() | nil} | :removed
```

Applies `event` (appended for `event_source_id`) to `instance` (or `nil`, for a
new instance) according to `read_model_module`'s `from/2` and `removed_with/2`
declarations.

Returns `{:ok, instance}`, `{:ok, nil}` when no `from/2` matches the event (the
instance is returned unchanged), or `:removed` when a `removed_with/2` matches.

---

*Consult [api-reference.md](api-reference.md) for complete listing*