Duplicating entry with belongs_to

Questions / Help
1

I am trying to duplicate an existing entry with a different :belongs_to value but am receiving the following error:

** (ArgumentError) cannot change belongs_to association item_type because there is already a change setting its foreign key item_type_id to 1

Here is the relevant code:
Migrations:

    create table(:item_type) do
      add :name, :text
    end

    create table(:item) do
      add :name, :text
      add :item_type_id, references(:item_type, on_delete: :nothing)
    end

Schema:

  schema "item_type" do
    field :name, :string
    has_many :items, Item
  end

  schema "item" do
    field :name, :string
    belongs_to :item_type, ItemType
  end

And the get/insert code that generates the error:

  def changeset(item, params \\ %{}) do
    item
    |> cast(params, [:name, :item_type_id])
    |> validate_required([:name])
  end

  def get_insert(attrs) do
    Repo.get(Item, attrs.id)
    |> Repo.preload(:item_type)
    |> Item.changeset(%{item_type_id: attrs.item_type_id})
    |> Repo.insert()
  end

The changeset and changeset.data generated by the above get_insert function:

#Ecto.Changeset<
  action: nil,
  changes: %{item_type_id: 1},
  errors: [],
  data: #Bevdb.Data.Item<>,
  valid?: true
>
%Bevdb.Data.Item{
  __meta__: #Ecto.Schema.Metadata<:loaded, "item">,
  id: 1,
  item_type: %Bevdb.Data.ItemType{
    __meta__: #Ecto.Schema.Metadata<:loaded, "item_type">,
    id: 2,
    items: #Ecto.Association.NotLoaded<association :items is not loaded>,
    name: "Type One"
  },
  item_type_id: 2,
  name: "Item One"
}

Any help is appreciated. I am new to Elixir and struggling a bit with associations.

Let me know if I can provide any additional information.

Thanks!

2

You do not need to preload the :item_type, and I suspect (without checking, sorry!), that this is the problem: it has preloaded the :item_type struct, but then it tries to change the :item_type_id which means that the id’s no longer match … and when it goes to Repo.insert the record it is trying to change both the item’s type id and insert an item_type record…

Try leaving out the the Repo.preload in get_insert and that should allow it to just set the :item_type_id on its own.

3

Thanks for the suggestion, but removing the preload gives the error:

** (RuntimeError) attempting to cast or change association item_type from Bevdb.Data.Item that was not loaded. Please preload your associations before manipulating them through changesets

4

Ah, right, because you are loading the struct from the DB first, and then not updating it be trying to re-insert the whole thing. So ecto is seeing an existing item and passing that to insert as if it were new …

Instead, create a new Item altogether. Call Map.from_struct on the loaded item, set the :item_type_id on that map, and then pass the whole thing into the changeset function … this will get you a new struct you can insert, sth like:

 def get_insert(attrs) do
    changes = Repo.get(Item, attrs.id)
      |> Map.from_struct()
      |> Map.put(:item_type_id, attrs.item_type_id)

    Item.changeset(%Item{}, changes)
    |> Repo.insert()
  end
5

Thank you! Your solution worked well.