How To Destroy/ Delete a Record With Many To Many Relationship? Error: would leave records behind

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1

Hi all,

I have a Item resource that has a many_to_many relationship with Category resource through ItemCategory resource.

I can create, update and read, but I am getting error: “would leave records behind” when trying to delete an Item or a Category. What’s the proper approach to deleting in this case?

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2

Hey,

I think you should have a look at reference relationship aspecialy on_delete:

https://hexdocs.pm/ash_postgres/dsl-ashpostgres-datalayer.html#postgres-references-reference

3

Thanks. Where in the resouce should I put it?

4

Here is an example: DSL: AshPostgres.DataLayer — ash_postgres v2.0.1

You will want to configure the join resource’s references to tell the system to delete the joins when their :item or :category is deleted.

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5

It is not clear whether the references are added within relationships block in a resource and whether this requires to generate a new migration.

Also it looks like I am missing a module import or requirement, but I can’t tell which one to import. I am getting the following error:

C:\laragon\www\elixir\kamaro_ops (Adopting-AshFramework -> origin)
λ iex -S mix phx.server
Compiling 2 files (.ex)
    error: undefined function references/1 (there is no such import)
    │
 27 │   references do
    │   ^
    │
    └─ lib/kamaro/stock/item_category.ex:27:3: Kamaro.Stock.ItemCategory (module)


== Compilation error in file lib/kamaro/stock/item_category.ex ==
** (CompileError) lib/kamaro/stock/item_category.ex: cannot compile module Kamaro.Stock.ItemCategory (errors have been logged)

Here is the join resource: ItemCategory

defmodule Kamaro.Stock.ItemCategory do
  use Ash.Resource,
      domain: Kamaro.Stock,
      data_layer: AshPostgres.DataLayer

  postgres do
    table "stock_item_categories"
    repo Kamaro.Repo
  end

  actions do
    defaults [:create, :read, :destroy]
  end

  relationships do
    belongs_to :item, Kamaro.Stock.Item do
      primary_key? true
      allow_nil? false
    end

    belongs_to :category, Kamaro.Stock.Category do
      primary_key? true
      allow_nil? false
    end
  end

  references do
    reference :item, on_delete: :delete, on_update: :update, name: "stock_item_categories_item_id_fkey"
  end
end
6

It is added in the postgre block and it does require generating migrations. I’d suggest familiarizing yourself with the structure of the DSL docs, as the DSL docs do show which section you’re in.

7

Thank you very much. I will follow your advise.

1 Like