# Aggregates

Aggregates in Ash allow for retrieving summary information over groups of related data. A simple example might be to show the "count of published posts for a user".

## Declaring aggregates on a resource

Example:

```elixir
aggregates do
  count :count_of_posts, :posts do
    filter expr(published == true)
  end
end
```

The available aggregate types are:

- `count` - counts related items meeting the criteria
- `first` - gets the first related value matching the criteria. Must specify the `field` to get.
- `sum` - sums the related items meeting the criteria. Must specify the `field` to sum.
- `list` - lists the related values. Must specify the `field` to list.

See the docs on {{link:ash:dsl:aggregates/aggregate}} for more information.

The aggregates declared on a resource allow for declaring a set of named aggregates that can be used by extensions.

As an escape hatch, they can also be loaded in the query using `Ash.Query.load/2`, or after the fact using `c:Ash.Api.load/3`. Aggregates declared on the resource will be keys in the resource's struct.

## Custom aggregates in the query

Example:

```elixir
User
|> Ash.Query.new()
|> Ash.Query.aggregate(:count_of_posts, :count, :posts, filter: [published: true])
```

See the documentation for `Ash.Query.aggregate/4` for more information.