Trouble with many to many relationship set by value

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1

I’m struggling trying to setup a many_to_many relationship for Tags. I’d like to create an action that updates the tags by providing the tag name and not the id . I’d like it to use existing tag records if they already exist with that name or create new ones if they don’t already exist. I have the relationship working but it keeps trying to re-create the same tags. Is there a way to get Ash to lookup existing tags using the name value?

Minimal version of my code:

defmodule Catalog.Tag do
  ...
  attributes do
    uuid_primary_key :id
    attribute :name, :string do
      allow_nil? false
    end
  end

  identities do
    identity :unique_tag, :name
  end
end

defmodule Catalog.ProductTag do
  ...

  attributes do
    uuid_primary_key :id
  end

  identities do
    identity :unique_product_tag, [:product_id, :tag_id]
  end

  relationships do
    belongs_to(:product, Shopkeeper.Catalog.Product)
    belongs_to(:tag, Shopkeeper.Catalog.Tag)
  end
end

defmodule Catalog.Product do
  ...

  actions do
    ...

    update :set_tags do
      accept([])
      argument(:tags, {:array, :map}, allow_nil?: false)
      change(
        manage_relationship(
          :tags,
          on_lookup: :relate,
          on_no_match: :create,
          on_match: :ignore,
          on_missing: :unrelate
        )
      )
    end
  end

  relationships do
    many_to_many(:tags, Catalog.Tag) do
      through Catalog.ProductTag
      source_attribute_on_join_resource :product_id
      destination_attribute_on_join_resource :tag_id
    end
  end
end

When running these commands I get the following error:

iex(6)> {:ok, product} = Product.set_tags(product, [%{name: "My Tag"}])

iex(7)> {:ok, product} = Product.set_tags(product, [%{name: "My Tag"}])

** (MatchError) no match of right hand side value: {:error, %Ash.Error.Invalid{errors: [%Ash.Error.Changes.InvalidAttribute{field: :product_id, message: "has already been taken", private_vars: [constraint: "product_tags_unique_product_tag_index", constraint_type: :unique], value: nil, changeset: nil, query: nil, error_context: [], vars: [], path: [], stacktrace: #Stacktrace<>, class: :invalid}], stacktraces?: true, changeset: #Ash.Changeset<api: Shopkeeper.Catalog, action_type: :create, action: :create, attributes: %{id: "23d907b5-e94f-4bae-b9bd-8f91a3f86c9a", product_id: "42bc8558-61e9-4c6f-9808-3b6aebe38064", tag_id: "6016b8d8-8198-4e6f-ab88-ce013dbb4f0b"}, relationships: %{}, errors: [%Ash.Error.Changes.InvalidAttribute{field: :product_id, message: "has already been taken", private_vars: [constraint: "product_tags_unique_product_tag_index", constraint_type: :unique], value: nil, changeset: nil, query: nil, error_context: [], vars: [], path: [], stacktrace: #Stacktrace<>, class: :invalid}], data: #Shopkeeper.Catalog.ProductTag<tag: #Ash.NotLoaded<:relationship>, product: #Ash.NotLoaded<:relationship>, __meta__: #Ecto.Schema.Metadata<:built, "product_tags">, id: nil, product_id: nil, tag_id: nil, aggregates: %{}, calculations: %{}, ...>, context: %{actor: nil, authorize?: false, accessing_from: %{name: :tags_join_assoc, source: Shopkeeper.Catalog.Product}}, valid?: false>, query: nil, error_context: [nil], vars: [], path: [], stacktrace: #Stacktrace<>, class: :invalid}}

I’d expect this to be a no-op instead as the tag has already been assigned.

2

https://hexdocs.pm/ash/Ash.Changeset.html#manage_relationship/4-options
You need to specify the :use_identities
option and tell it to use the :unique_tag identity

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