# Commanded Ecto projections
Read model projections for [Commanded](https://github.com/slashdotdash/commanded) CQRS/ES applications using [Ecto](https://github.com/elixir-ecto/ecto) for persistence.
MIT License
[](https://travis-ci.org/slashdotdash/commanded-ecto-projections)
---
## Installation
You should already have [Ecto](https://github.com/elixir-ecto/ecto) installed and configured before proceeding. Please follow the Ecto [Getting Started](https://hexdocs.pm/ecto/getting-started.html) guide to get going first.
1. Add `commanded_ecto_projections` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:commanded_ecto_projections, "~> 0.3"},
]
end
```
2. Configure `commanded_ecto_projections` with the Ecto repo used by your application:
```elixir
config :commanded_ecto_projections,
repo: MyApp.Projections.Repo
```
3. Generate an Ecto migration in your app:
```console
mix ecto.gen.migration create_projection_versions
```
4. Modify the generated migration, in `priv/repo/migrations`, to create the `projection_versions` table:
```elixir
defmodule CreateProjectionVersions do
use Ecto.Migration
def change do
create table(:projection_versions, primary_key: false) do
add :projection_name, :text, primary_key: true
add :last_seen_event_number, :bigint
timestamps()
end
end
end
```
4. Run the Ecto migration:
```console
mix ecto.migrate
```
## Usage
Use Ecto schemas to define your read model:
```elixir
defmodule ExampleProjection do
use Ecto.Schema
schema "example_projections" do
field :name, :string
end
end
```
For each read model you will need to define a module that uses the `Commanded.Projections.Ecto` macro and configures the domain events to be projected.
The `project/2` macro expects the domain event and metadata. You can also use `project/1` if you do not need to use the event metadata. Inside the project block you have access to an [Ecto.Multi](https://hexdocs.pm/ecto/Ecto.Multi.html) data structure, available as the `multi` variable, for grouping multiple Repo operations. These will all be executed within a single transaction. You can use `Ecto.Multi` to insert, update, and delete data.
```elixir
defmodule MyApp.ExampleProjector do
use Commanded.Projections.Ecto, name: "example_projection"
project %AnEvent{name: name}, _metadata do
Ecto.Multi.insert(multi, :example_projection, %ExampleProjection{name: name})
end
project %AnotherEvent{name: name} do
Ecto.Multi.insert(multi, :example_projection, %ExampleProjection{name: name})
end
end
```
### Supervision
Your projector module must be included in your application supervision tree:
```elixir
defmodule MyApp.Projections.Supervisor do
use Supervisor
def start_link do
Supervisor.start_link(__MODULE__, nil)
end
def init(_) do
children = [
# projections
worker(MyApp.ExampleProjector, [], id: :projector),
]
supervise(children, strategy: :one_for_one)
end
end
```
### Rebuilding a projection
The `projection_versions` table is used to ensure that events are only projected once.
To rebuild a projection you will need to:
1. Delete the row containing the last seen event for the projection name:
```SQL
delete from projection_versions
where projection_name = 'example_projection';
```
2. Truncate the tables that are being populated by the projection, and restart their identity:
```SQL
truncate table
example_projections,
other_projections
restart identity;
```
You will also need to reset the event store subscription for the commanded event handler. This is specific to whichever event store you are using.
### Contributing
Pull requests to contribute new or improved features, and extend documentation are most welcome. Please follow the existing coding conventions.
You should include unit tests to cover any changes. Run `mix test` to execute the test suite:
```console
mix deps.get
MIX_ENV=test mix do ecto.create, ecto.migrate
mix test
````
