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1.3.37
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1.3.34
1.3.33
1.3.32
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1.3.30
1.3.29
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0.1.0
The PostgreSQL data layer for Ash Framework
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documentation/topics/resources/polymorphic-resources.md
<!--
SPDX-FileCopyrightText: 2020 Zach Daniel
SPDX-License-Identifier: MIT
-->
# 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:
```elixir
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:
```elixir
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:
```elixir
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`.