polymorphic-resources.md

Polymorphic Resources

To support leveraging the same resource backed by multiple tables (useful for things like polymorphic associations), AshPostgres supports setting the data_layer.table context for a given resource. For this example, lets assume that you have a MyApp.Post resource and a MyApp.Comment resource. For each of those resources, users can submit reactions. However, you want a separate table for post_reactions and comment_reactions. You could accomplish that like so:

defmodule MyApp.Reaction do
  use Ash.Resource,
    domain: MyDomain,
    data_layer: AshPostgres.DataLayer

  postgres do
    polymorphic? true # Without this, `table` is a required configuration
  end

  attributes do
    attribute :resource_id, :uuid, public?: true
  end

  ...
end

Then, in your related resources, you set the table context like so:

defmodule MyApp.Post do
  use Ash.Resource,
    domain: MyDomain,
    data_layer: AshPostgres.DataLayer

  ...

  relationships do
    has_many :reactions, MyApp.Reaction,
      relationship_context: %{data_layer: %{table: "post_reactions"}},
      destination_attribute: :resource_id
  end
end

defmodule MyApp.Comment do
  use Ash.Resource,
    domain: MyDomain,
    data_layer: AshPostgres.DataLayer

  ...

  relationships do
    has_many :reactions, MyApp.Reaction,
      relationship_context: %{data_layer: %{table: "comment_reactions"}},
      destination_attribute: :resource_id
  end
end

With this, when loading or editing related data, ash will automatically set that context. For managing related data, see Ash.Changeset.manage_relationship/4 and other relationship functions in Ash.Changeset

Table specific actions

To make actions use a specific table, you can use the set_context query preparation/change.

For example:

defmodule MyApp.Reaction do
  # ...
  actions do
    read :for_comments do
      prepare set_context(%{data_layer: %{table: "comment_reactions"}})
    end

    read :for_posts do
      prepare set_context(%{data_layer: %{table: "post_reactions"}})
    end
  end
end

Migrations

When a migration is marked as polymorphic? true, the migration generator will look at all resources that are related to it, that set the %{data_layer: %{table: "table"}} context. For each of those, a migration is generated/managed automatically. This means that adding reactions to a new resource is as easy as adding the relationship and table context, and then running mix ash.codegen.