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Read Actions

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

# 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

For a full list of all of the available options for configuring read actions, see the Ash.Resource.Dsl documentation.

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:

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.

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.

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
  • Apply pagination 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