Topics

Ash Framework•3y ago

Polymorphic resources

I have a number of resources that I'm beginning to add things like comments, attachments to, and would be interested what the idiomatic way to do this is. For this case, I can see a macro or something like https://discord.com/channels/711271361523351632/1079057460700123186/1092860018862346300 working as a template, specifying a different resource name and table for each instance.

49 Replies

ZachDaniel•3y ago

I would suggest writing an extension for that. Fragments are not meant for sharing behavior among resources, but for splitting up single resources. I can show you what an extension might look like for something like that when I’m back at my computer.

\ ឵឵឵OP•3y ago

That would be lovely, thanks mate!

ZachDaniel•3y ago

What you might also actually do is use ash_postgres' polymorphic? true option

ZachDaniel•3y ago

https://ash-hq.org/docs/guides/ash_postgres/latest/topics/polymorphic_resources

Ash HQ

Guide: Polymorphic Resources

Read the "Polymorphic Resources" guide on Ash HQ

ZachDaniel•3y ago

This lets you have a single resource for MyApp.Comment for example, but have it create/manage many tables. So then you'd do:

has_many :comments, MyApp.Comment do
  context %{data_layer: %{table: "post_comments"}}
end

has_many :comments, MyApp.Comment do
  context %{data_layer: %{table: "post_comments"}}
end

The migration generator will find all relationships to polymorphic resources and generate a table for every single one of them.

\ ឵឵឵OP•3y ago

Ok, interesting Looks quite nice, seems like the one major tradeoff would be sacrificing the reverse relationship.

ZachDaniel•3y ago

Yeah, that is correct.

\ ឵឵឵OP•3y ago

Is there a way with Spark to write an extension that is similarly terse to a fragment? If not, any downsides to combining extension + fragment for that purpose?

ZachDaniel•3y ago

Well, the fragment won't have any dynamism So you couldn't do things like modify the keys i.e assuming destination_attribute :post_id on one resource and destination_attribute :other_thing_id on one resource Honestly this is probably not worth it, but: The alternative, done with an extension, might look something like this:

defmodule MyApp.Extensions.Commentable do
  @comments %Spark.Dsl.Section{
    name: :comments,
    schema: [
      # may want more options here
      reverse_relationship: [
        type: :atom,
        required: true
      ],
      table: [
        type: :string
      ]
    ]
  }

 @transformers [MyApp.Extensions.Commentable.Transformers.AddComments]

  use Spark.Dsl.Extension, sections: [@comments], transformers: @transformers
end

defmodule MyApp.Extensions.Commentable.Info do
  # Generate introspection functions
  use Spark.InfoGenerator, extensions: MyApp.Extensions.Commentable, sections: [:comments]
end

defmodule MyApp.Extensions.Commentable.Transformers.AddComments do
  use Spark.Dsl.Transformer
  alias Spark.Dsl.Transformer

  def transform(dsl) do
    comments_resource = define_comments_resource(dsl)

    Ash.Resource.Builder.add_relationship(dsl, :has_many, :comments, comments_resource)
  end

  defp define_comments_resource(dsl) do
    resource_module = Transformer.get_persisted(dsl, :module)
    comments_module = Module.concat(resource_module, Comments)

    defmodule comments_module do
       use Ash.Resource,
         data_layer: AshPostgres.DataLayer
        
       postgres do
         table unquote(MyApp.Extensions.Commentable.Info.comments_table(dsl) || "#{AshPostgres.DataLayer.Info.table(dsl)}_comments")
         repo MyApp.Repo
       end

      attributes do
        attribute :text, :string, allow_nil?: false
      end

      relationships do
        belongs_to unquote(MyApp.Extensions.Commentable.Info.comments_reverse_relationship(dsl)), unquote(comments_module) do
          allow_nil? false
        end
      end
    end

    comments_module
  end
end

defmodule MyApp.Extensions.Commentable do
  @comments %Spark.Dsl.Section{
    name: :comments,
    schema: [
      # may want more options here
      reverse_relationship: [
        type: :atom,
        required: true
      ],
      table: [
        type: :string
      ]
    ]
  }

 @transformers [MyApp.Extensions.Commentable.Transformers.AddComments]

  use Spark.Dsl.Extension, sections: [@comments], transformers: @transformers
end

defmodule MyApp.Extensions.Commentable.Info do
  # Generate introspection functions
  use Spark.InfoGenerator, extensions: MyApp.Extensions.Commentable, sections: [:comments]
end

defmodule MyApp.Extensions.Commentable.Transformers.AddComments do
  use Spark.Dsl.Transformer
  alias Spark.Dsl.Transformer

  def transform(dsl) do
    comments_resource = define_comments_resource(dsl)

    Ash.Resource.Builder.add_relationship(dsl, :has_many, :comments, comments_resource)
  end

  defp define_comments_resource(dsl) do
    resource_module = Transformer.get_persisted(dsl, :module)
    comments_module = Module.concat(resource_module, Comments)

    defmodule comments_module do
       use Ash.Resource,
         data_layer: AshPostgres.DataLayer
        
       postgres do
         table unquote(MyApp.Extensions.Commentable.Info.comments_table(dsl) || "#{AshPostgres.DataLayer.Info.table(dsl)}_comments")
         repo MyApp.Repo
       end

      attributes do
        attribute :text, :string, allow_nil?: false
      end

      relationships do
        belongs_to unquote(MyApp.Extensions.Commentable.Info.comments_reverse_relationship(dsl)), unquote(comments_module) do
          allow_nil? false
        end
      end
    end

    comments_module
  end
end

So that actually defines the comments module for the resource automatically.

use Ash.Resource, extensions: [MyApp.Extensions.Commentable]

comments do
  reverse_relationship :post
end

use Ash.Resource, extensions: [MyApp.Extensions.Commentable]

comments do
  reverse_relationship :post
end

Would be how you configure it in a given resource

\ ឵឵឵OP•3y ago

Very cool! That looks pretty reasonable, but I understand what you mean that it might be a bit overkill.

ZachDaniel•3y ago

It really just depends on how bought into the idea of constructing your app from your core domain + extensions 😆 I'd do the above, because I know how it all works

\ ឵឵឵OP•3y ago

My thought was to hit somewhere in the middle and have an extension for the comments resource itself that would essentially do the same polymorphism as the data layer approach above.

ZachDaniel•3y ago

Yeah, you can definitely do that So youd do something like:

defmodule MyApp.PostComments do
  use Ash.Resource, extensions: [MyApp.Extensions.Comment]

  comment do
    comments_on :post, MyApp.Post
  end
end

defmodule MyApp.PostComments do
  use Ash.Resource, extensions: [MyApp.Extensions.Comment]

  comment do
    comments_on :post, MyApp.Post
  end
end

?

\ ឵឵឵OP•3y ago

If I were to write it as a macro, something like:

defmodule App.Comment do
  def __using__(opts \\ []) do
    name = opts[:name]
    quote do
      use Ash.Resource, ...

      attributes do
        attribute :content, :string
        ...
      end

      relationships do
        belongs_to :parent, unquote(opts[:parent])
      end

      postgres do
        table unquote(opts[:name])
      end
    end
  end
end

defmodule App.PostComment do
  use App.Comment, name: "post_comments", parent: App.Post
end

defmodule App.Comment do
  def __using__(opts \\ []) do
    name = opts[:name]
    quote do
      use Ash.Resource, ...

      attributes do
        attribute :content, :string
        ...
      end

      relationships do
        belongs_to :parent, unquote(opts[:parent])
      end

      postgres do
        table unquote(opts[:name])
      end
    end
  end
end

defmodule App.PostComment do
  use App.Comment, name: "post_comments", parent: App.Post
end

The extension definitely does a bit more for you.

ZachDaniel•3y ago

That will work, up to a point. If you try to use expr in there i.e to add actions that do certain things (not that you need to) then you may see some issues. Additionally, you won't be able to benefit from some compile time optimizations we make available to extensions. Its definitely okay to do what you've shown there. But FWIW I encourage people to "break the seal" so to speak on writing extensions as early as possible, because they are the most flexible way to extend your Ash app, and come with lots of tools to solve common metaprogramming problems. But its totally up to you, and the above will likely work just fine 🙂

\ ឵឵឵OP•3y ago

Sure thing! I have a number of macros that I would consider converting to Spark, I think it would also benefit error reporting and validation. Actually, I was curious how one would write the above macro using Spark. In this particular case, it's not a problem for comments/attachments to be defined in the parent resource (except making sure that GQL doesn't get upset), so the comments section is quite cool. This would also make it easy to add actions on that resource in the parent.

ZachDaniel•3y ago

You can do all sorts of things in spark transformers, like set DSL options, add entities (attributes/relationships) that kind of thing its only partially automated. I.e you have Ash.Resource.Builder.add_attribute but for setting the table you'd need to do something like Transformer.set_option(dsl, [:postgres], :table, "...")

\ ឵឵឵OP•3y ago

No worries, that makes sense. Is there a generic helper like that for appending to a section that supports multiple, or is that the behaviour of set_option already? Starting to test out the Commentable modules, pretty interesting stuff. Were the unquotes intentional?

ZachDaniel•3y ago

For your first question you'd use Transformer.add_entity (or ideally the functions in Ash.Resource.Builder) there are sections, entities, and options

\ ឵឵឵OP•3y ago

Unfortunately looks like I may be running into problems mixing my existing macros with the extensions, as there's stuff defined there that this one wants.

ZachDaniel•3y ago

resource do <- section
  description "" <- option
end

attributes do <- section
  attribute :foo, :bar <- entity
end

resource do <- section
  description "" <- option
end

attributes do <- section
  attribute :foo, :bar <- entity
end

The unquote was intentional, yes

\ ឵឵឵OP•3y ago

It was erroring out for me on dsl not being defined.

ZachDaniel•3y ago

Ah, sorry you need:

Module.create(module_name, quote do
  #the module body
end, __ENV__)

Module.create(module_name, quote do
  #the module body
end, __ENV__)

\ ឵឵឵OP•3y ago

After removing them it seems to be doing better, but I derive my table name from the module in my base resource macro so it doesn't seem to be picked up.

ZachDaniel•3y ago

I don't think thats the issue

\ ឵឵឵OP•3y ago

Ah, right on. That makes more sense.

ZachDaniel•3y ago

the table name should be set by the time you are in the transformer

\ ឵឵឵OP•3y ago

Even when defined in a use macro?

ZachDaniel•3y ago

yep

\ ឵឵឵OP•3y ago

I thought so as well... Then there must be something else going on.

** (RuntimeError) No configuration for `table` present on `%{:persist => %{autho

** (RuntimeError) No configuration for `table` present on `%{:persist => %{autho

I see other stuff generated by the macro in that struct, though.

ZachDaniel•3y ago

That error looks strange Can you paste the whole transformer in?

\ ឵឵឵OP•3y ago

defmodule App.Extension.Commentable do
  @comments %Spark.Dsl.Section{
    name: :comments,
    schema: [
      # may want more options here
      reverse_relationship: [
        type: :atom,
        required: true
      ],
      table: [
        type: :string
      ]
    ]
  }

  @transformers [App.Extension.Commentable.Transformers.AddComments]

  use Spark.Dsl.Extension, sections: [@comments], transformers: @transformers
end

defmodule App.Extension.Commentable.Info do
  # Generate introspection functions
  use Spark.InfoGenerator, extension: App.Extension.Commentable, sections: [:comments]
end

defmodule App.Extension.Commentable.Transformers.AddComments do
  use Spark.Dsl.Transformer
  alias Spark.Dsl.Transformer

  def transform(dsl) do
    comments_resource = define_comments_resource(dsl)

    Ash.Resource.Builder.add_relationship(dsl, :has_many, :comments, comments_resource)
  end

  defp define_comments_resource(dsl) do
    resource_module = Transformer.get_persisted(dsl, :module)
    comments_module = Module.concat(resource_module, Comments)
    table_name = App.Extension.Commentable.Info.comments_table!(dsl) ||
      "#{AshPostgres.DataLayer.Info.table(dsl)}_comments"
    reverse_relationship = App.Extension.Commentable.Info.comments_reverse_relationship!(dsl)

    Module.create(comments_module, quote do
      use Ash.Resource, data_layer: AshPostgres.DataLayer

      postgres do
        table unquote(table_name)
        repo App.Repo
      end

      attributes do
        attribute :text, :string, allow_nil?: false
      end

      relationships do
        belongs_to unquote(reverse_relationship), unquote(resource_module) do
          allow_nil? false
        end
      end
    end, __ENV__)

    comments_module
  end
end

defmodule App.Extension.Commentable do
  @comments %Spark.Dsl.Section{
    name: :comments,
    schema: [
      # may want more options here
      reverse_relationship: [
        type: :atom,
        required: true
      ],
      table: [
        type: :string
      ]
    ]
  }

  @transformers [App.Extension.Commentable.Transformers.AddComments]

  use Spark.Dsl.Extension, sections: [@comments], transformers: @transformers
end

defmodule App.Extension.Commentable.Info do
  # Generate introspection functions
  use Spark.InfoGenerator, extension: App.Extension.Commentable, sections: [:comments]
end

defmodule App.Extension.Commentable.Transformers.AddComments do
  use Spark.Dsl.Transformer
  alias Spark.Dsl.Transformer

  def transform(dsl) do
    comments_resource = define_comments_resource(dsl)

    Ash.Resource.Builder.add_relationship(dsl, :has_many, :comments, comments_resource)
  end

  defp define_comments_resource(dsl) do
    resource_module = Transformer.get_persisted(dsl, :module)
    comments_module = Module.concat(resource_module, Comments)
    table_name = App.Extension.Commentable.Info.comments_table!(dsl) ||
      "#{AshPostgres.DataLayer.Info.table(dsl)}_comments"
    reverse_relationship = App.Extension.Commentable.Info.comments_reverse_relationship!(dsl)

    Module.create(comments_module, quote do
      use Ash.Resource, data_layer: AshPostgres.DataLayer

      postgres do
        table unquote(table_name)
        repo App.Repo
      end

      attributes do
        attribute :text, :string, allow_nil?: false
      end

      relationships do
        belongs_to unquote(reverse_relationship), unquote(resource_module) do
          allow_nil? false
        end
      end
    end, __ENV__)

    comments_module
  end
end

Interestingly, if I set table "posts_comments" explicitly, and add uuid_primary_key :id, now i'm getting:

warning: invalid association `issue` in schema App.Post.Comments: associated module App.Post is not an Ecto schema
  lib/uncle/extensions/commentable.ex:61: App.Post.Comments (module)

warning: invalid association `issue` in schema App.Post.Comments: associated module App.Post is not an Ecto schema
  lib/uncle/extensions/commentable.ex:61: App.Post.Comments (module)

ZachDaniel•3y ago

App.Extension.Commentable.Info.comments_table!(dsl) that raises an error if the configuration is not set IIRC You might need

case App.Extesion.Commentable.Info.comments_table(dsl) do
  {:ok, table} -> table
  :error -> <default>
end

case App.Extesion.Commentable.Info.comments_table(dsl) do
  {:ok, table} -> table
  :error -> <default>
end

\ ឵឵឵OP•3y ago

    table_name =
      case App.Extension.Commentable.Info.comments_table(dsl) do
        {:ok, name} -> name
        :error -> "#{AshPostgres.DataLayer.Info.table(dsl)}_comments"
      end

    table_name =
      case App.Extension.Commentable.Info.comments_table(dsl) do
        {:ok, name} -> name
        :error -> "#{AshPostgres.DataLayer.Info.table(dsl)}_comments"
      end

Probably need ! for Info.table as well. Ok, still getting the warning about not being an Ecto schema, but now a ton of GQL complaints about non-unique types. Fixed the GQL stuff 🙂 Can I discover the registry of the parent with Spark and have the comment resource add itself?

ZachDaniel•3y ago

only kind of What you actually have to do is write an extension for the registry 😆

defmodule RegistryExtension do
  use Spark.Dsl.Extension, transformers: [AddCommentsResource]
end

defmodule AddCommentsResource do
  use Spark.Dsl.Transformer
  alias Spark.Dsl.Transformer

  # You want this to go before everything else
  def before?(_), do: true

  def transform(dsl) do
    resources = Ash.Registry.entries(dsl)
    # use `Transformer.add_entity` for each module using your extension
  end
end

defmodule RegistryExtension do
  use Spark.Dsl.Extension, transformers: [AddCommentsResource]
end

defmodule AddCommentsResource do
  use Spark.Dsl.Transformer
  alias Spark.Dsl.Transformer

  # You want this to go before everything else
  def before?(_), do: true

  def transform(dsl) do
    resources = Ash.Registry.entries(dsl)
    # use `Transformer.add_entity` for each module using your extension
  end
end

\ ឵឵឵OP•3y ago

Nice, will add that in as well. Cool! Is there a more optimal way to detect the comments section on the resource than

      case App.Extension.Commentable.Info.comments_reverse_relationship(r) do

      case App.Extension.Commentable.Info.comments_reverse_relationship(r) do

ZachDaniel•3y ago

You can do YourExtensionName in Spark.extensions(module)

\ ឵឵឵OP•3y ago

Right on. I thought about just adding the extension to my base resource so I wanted to check if the section is used. Is there a way to detect the presence of the whole comments section rather than an entry in it? Or is it there implicitly and empty? Tbh this is probably not the normal pattern 🙂

ZachDaniel•3y ago

Yeah, I wouldn't suggest doing that There is no good way to detect if a section has been opened

\ ឵឵឵OP•3y ago

Cool

ZachDaniel•3y ago

If at all possible, I'd suggest just adding your extension to each resource, but otherwise you'd need to add some kind of boolean flag like

comments do
  enabled? true
end

comments do
  enabled? true
end

Welll...I guess maybe that isn't true I think you can do Map.has_key?(dsl, [:comments])? still, would be pretty non-idiomatic

\ ឵឵឵OP•3y ago

For sure, I'll stick with letting the existence of the extension be the indicator. Thanks a lot, mate! Will definitely be looking for opportunities to use this going forward 🙂 Wanted to check back in on this one. I'm still seeing this warning:

warning: invalid association `post` in schema App.Post.Comment: associated module App.Post is not an Ecto schema

warning: invalid association `post` in schema App.Post.Comment: associated module App.Post is not an Ecto schema

Any idea what might be the cause?

ZachDaniel•3y ago

🤔 Interesting. So App.Post is the target Ash resource?

\ ឵឵឵OP•3y ago

App.Post is the resource with the comments block.

ZachDaniel•3y ago

🤔 but everything works otherwise? If so, mind making an issue in Ash? I'm not sure there is anything we can actually do about it but I can look into it Well, there is probably something we can do 🙂

\ ឵឵឵OP•3y ago

Yep, will do when I get back to my desk. Any idea what the issue might be?

ZachDaniel•3y ago

The basic issue is that, as a part of compiling App.Post you are compiling App.Post.Comment which then refers back to App.Post. Which is fine, except for ecto providing that warning.

\ ឵឵឵OP•3y ago

https://github.com/ash-project/ash/issues/557

GitHub

Resources defined within other resources issue Ecto warnings · Issu...

When creating a subresource using an extension, the compiler issues the following warning: warning: invalid association post in schema App.Post.Comment: associated module App.Post is not an Ecto ...

ZachDaniel•3y ago

🙇

Ash Framework Join

The Elixir backend framework for unparalleled productivity. Declarative tools that let you stop wasting time. Use with Phoenix LiveView or build APIs in minutes for your front-end of choice.

2KMembers

View on Discord

Did you find this page helpful?