How do I use Ecto cast_embed correctly on this Map?

Phoenix Forum Questions / Help
, ,
1

I’m trying to figure out how to use cast_embed but I don’t understand what I’m doing wrong.

defmodule Operation do
  use Ecto.Schema

  embedded_schema do
    field :amount, :integer
    field :type, :string
    embeds_one :item, Item
  end
end

defmodule Item do
  use Ecto.Schema

  embedded_schema do
    field :item_code, :string
    field :item_name, :string
  end
end

with the following data

{ 
  "item": {
    "item_code": "A1",
    "item_name": "NameA1"
  },
  "type": "type",
  "amount": 4
}

d = %{
  "amount" => 4,
  "item" => %{"item_code" => "A1", "item_name" => "NameA1"},
  "type" => "type"
}

cast(%Operation{}, d, [:amount, :type]) |> validate_required([:amount, :type]) |> cast_embed(:item)

This throws an error:

** (UndefinedFunctionError) function Item.changeset/2 is undefined or private. Did you mean one of:

  * __changeset__/0

Item.changeset(%Item{id: nil, item_code: nil, item_name: nil}, %{"item_code" => "A1", "item_name" => "NameA1"})
(ecto) lib/ecto/changeset.ex:736: anonymous fn/4 in Ecto.Changeset.on_cast_default/2
(ecto) lib/ecto/changeset/relation.ex:111: Ecto.Changeset.Relation.do_cast/5
(ecto) lib/ecto/changeset/relation.ex:292: Ecto.Changeset.Relation.single_change/5
(ecto) lib/ecto/changeset.ex:715: Ecto.Changeset.cast_relation/4

Been stuck on this for a while and I can’t find a working example of how to do this online, any ideas what I’m doing wrong here?

1 Like
2

Take a look at the documentation for cast_embed

Specifically, note the :with option which states

:with - the function to build the changeset from params. Defaults to the changeset/2 function in the embed module

Basically, when you call cast_embed, it implicitly calls Item.changeset/2, but since you don’t have one defined its erroring out. You want to add a def changeset(changeset, params) function in your Item module

1 Like
3

Thanks, can you give an example of how that would be done with my above structs?
I’m still trying to get it working but frustratingly just getting errors…

4

This post is a cast_assoc example but should apply to cast_embed. Check it out:

Best regards,

2 Likes
5

you want something like

defmodule Item do
  use Ecto.Schema

  embedded_schema do
    field :item_code, :string
    field :item_name, :string
  end

  def changeset(schema, params) do
    schema
    |> cast(params, [:item_code, :item_name])
  end
end
3 Likes
6

Many thanks, that helped immensely and I get the gist of how it works now!