# Read Actions

Read actions operate on an `Ash.Query`. Read actions always return lists of data. The act of [pagination](#pagination), or returning a [single result](#ash-get), is handled as part of the interface, and is not a concern of the action itself. Here is an example of a read action:

```elixir
# Giving your actions informative names is always a good idea
read :ticket_queue do
  # Use arguments to take in values you need to run your read action.
  argument :priorities, {:array, :atom} do
    constraints items: [one_of: [:low, :medium, :high]]
  end

  # This action may be paginated,
  # and returns a total count of records by default
  pagination offset: true, countable: :by_default

  # Arguments can be used in preparations and filters
  filter expr(status == :open and priority in ^arg(:priorities))
end
```

## Ash.get!

The `Ash.get!` function is a convenience function for running a read action, filtering by a unique identifier, and expecting only a single result. It is equivalent to the following code:

```elixir
Ash.get!(Resource, 1)

# is roughly equivalent to

Resource
|> Ash.Query.filter(id == 1)
|> Ash.Query.limit(2)
|> Ash.read!()
|> case do
  [] -> # raise not found error
  [result] -> result
  [_, _] -> # raise too many results error
end
```

## Ash.read_one!

The `Ash.read_one!` function is a similar convenience function to `Ash.get!`, but it does not take a unique identifier. It is useful when you expect an action to return only a single result, and want to enforce that and return a single result.

```elixir
Ash.read_one!(query)

# is roughly equivalent to

query
|> Ash.Query.limit(2)
|> Ash.read!()
|> case do
  [] -> nil
  [result] -> result
  [_, _] -> # raise too many results error
end
```

## Pagination

Ash provides built-in support for pagination when reading resources and their relationships. You can find more information about this in the [pagination guide](/documentation/topics/advanced/pagination.livemd).

## What happens when you call Ash.Query.for_read/4

The following steps are performed when you call `Ash.Query.for_read/4`.

- Cast input arguments - `d:Ash.Resource.Dsl.actions.read.argument`
- Set default argument values - `d:Ash.Resource.Dsl.actions.read.argument|default`
- Add errors for missing required arguments | `d:Ash.Resource.Dsl.actions.read.argument|allow_nil?`
- Run query preparations | `d:Ash.Resource.Dsl.actions.read.prepare`
- Add action filter | `d:Ash.Resource.Dsl.actions.read|filter`

## What happens when you run the action

These steps are trimmed down, and are aimed at helping users understand the general flow. Some steps are omitted.

- Run `Ash.Query.for_read/3` if it has not already been run
- [Apply tenant filters for attribute](/documentation/topics/advanced/multitenancy.md)
- Apply [pagination](/documentation/topics/advanced/pagination.livemd) options
- Run before action hooks
- Multi-datalayer filter is synthesized. We run queries in other data layers to fetch ids and translate related filters to `(destination_field in ^ids)`
- Strict Check & Filter Authorization is run
- Data layer query is built and validated
- Field policies are added to the query
- Data layer query is Run
- Authorizer "runtime" checks are run (you likely do not have any of these)

The following steps happen while(asynchronously) or after the main data layer query has been run

- If paginating and count was requested, the count is determined at the same time as the query is run.
- Any calculations & aggregates that were able to be run outside of the main query are run
- Relationships, calculations, and aggregates are loaded