Ash Elixir•3y ago

Many To Many Relationship Example

After reading https://www.ash-hq.org/docs/guides/ash/latest/topics/relationships.md#has-many and https://ash-hq.org/docs/dsl/ash/latest/ash-resource-dsl/relationships/many_to_many I am a little bit confused since the examples use different code. I am looking for a simple example which I can copy and paste. Something like a blog post which has multiple tags associated via a many to many relationship:

defmodule Example.Blog.Post do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id

    attribute :content, :string do
      allow_nil? false
    end
  end
  
  postgres do
    table "blog_posts"
    repo Example.Repo
  end
  
  actions do
    defaults [:create, :read]
  end
end

defmodule Example.Blog.Post do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id

    attribute :content, :string do
      allow_nil? false
    end
  end
  
  postgres do
    table "blog_posts"
    repo Example.Repo
  end
  
  actions do
    defaults [:create, :read]
  end
end

defmodule Example.Blog.Tag do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id

    attribute :name, :string do
      allow_nil? false
    end
  end
  
  postgres do
    table "tags"
    repo Example.Repo
  end
  
  actions do
    defaults [:create, :read]
  end
end

defmodule Example.Blog.Tag do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id

    attribute :name, :string do
      allow_nil? false
    end
  end
  
  postgres do
    table "tags"
    repo Example.Repo
  end
  
  actions do
    defaults [:create, :read]
  end
end

- What do I have to change in the existing files? - How do I have to do the third resource? - How would I add a tag to a blog post the Ash way?

3 Replies

ZachDaniel•3y ago

There should probably be a guide specifically for this. Here is how I'd typically set it up:

defmodule Example.Blog.PostTag do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id
  end

  identities do
    identity :unique_post_tag, [:post_id, :tag_id]
  end

  relationships do
    belongs_to :post, Example.Blog.Post do
      allow_nil? false
    end

    belongs_to :tag, Example.Blog.Tag do
      allow_nil? false
    end
  end
end

# in your Post resource

relationships do
  many_to_many :tags, Example.Blog.Tag do
    through Example.Blog.PostTag
    source_attribute_on_join_resource :post_id
    source_attribute_on_join_resource :tag_id
  end
end

defmodule Example.Blog.PostTag do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id
  end

  identities do
    identity :unique_post_tag, [:post_id, :tag_id]
  end

  relationships do
    belongs_to :post, Example.Blog.Post do
      allow_nil? false
    end

    belongs_to :tag, Example.Blog.Tag do
      allow_nil? false
    end
  end
end

# in your Post resource

relationships do
  many_to_many :tags, Example.Blog.Tag do
    through Example.Blog.PostTag
    source_attribute_on_join_resource :post_id
    source_attribute_on_join_resource :tag_id
  end
end

Stefan WintermeyerOP•3y ago

I can write a tutorial for this or enrich the examples in the current docs. Let's have that discussion in #documentation Bonus question: What is "the Ash" way of adding a Tag to a Post? I know how to do this in Ecto but I am still unsure how an Ash developer tackles this best.

ZachDaniel•3y ago

There are two ways: manage_relationship and a custom change. manage_relationship:

update :add_tag do
  accept []

  argument :tag_name, :string do
    allow_nil? false
  end

  change manage_relationship(
    :tag_name, 
    :tags,
    on_lookup: :relate,
    on_no_match: :create
  )
end

update :add_tag do
  accept []

  argument :tag_name, :string do
    allow_nil? false
  end

  change manage_relationship(
    :tag_name, 
    :tags,
    on_lookup: :relate,
    on_no_match: :create
  )
end

custom change

update :add_tag do
  accept []
  
  argument :tag_name, :string do
    allow_nil? false
  end

  change fn changeset, _ -> 
    Ash.Changeset.after_action(fn changeset, post -> 
      tag_name = Ash.Changeset.get_argument(changeset, :tag_name)
      # assuming the create code interface on tag
      # (you can also make a dedicated upsert action)
      tag = Example.Blog.Tag.create!(tag_name, upsert?: true, upsert_identity: :unique_name)
      # assuming a create code interface on blog_tag
      Example.Blog.BlogTag.create!(post.id, tag.id)
      {:ok, post}
    end)
  end
end

update :add_tag do
  accept []
  
  argument :tag_name, :string do
    allow_nil? false
  end

  change fn changeset, _ -> 
    Ash.Changeset.after_action(fn changeset, post -> 
      tag_name = Ash.Changeset.get_argument(changeset, :tag_name)
      # assuming the create code interface on tag
      # (you can also make a dedicated upsert action)
      tag = Example.Blog.Tag.create!(tag_name, upsert?: true, upsert_identity: :unique_name)
      # assuming a create code interface on blog_tag
      Example.Blog.BlogTag.create!(post.id, tag.id)
      {:ok, post}
    end)
  end
end

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