elixir - Ecto - how to make assoc_constraint return the same error as validate_required? -

let's have model post belongs_to category:

defmodule myapp.post   use myapp.web, :model    schema "posts"     field :title, :string     belongs_to :category, myapp.category   end    def changeset(model, params)     model     |> cast(params, [:title, :category_id)     |> validate_required([:title, :category_id])     |> assoc_constraint(:category)   end end 

the problem is: if don't pass category_id params, changeset error key is: category_id, on assoc_constraint invalidation (if category isn't exist), have category key. it's little bit contrintuitive me - because problem same - there no category post. how can deal that?

the way find after reading through ecto's source give name want in error field assoc_constraint, , override constraint name yourself. default name belongs_to constraint #{table name}_#{column in table}_fkey (source).

edit: might use foreign_key_constraint if we're passing :name ourselves that's thing assoc_constraint foreign_key_constraint doesn't. i've updated code below.

migration:

defmodule myapp.repo.migrations.createcomment   use ecto.migration    def change     create table(:comments)       add :post_id, references(:posts, on_delete: :nothing)       timestamps()     end     create index(:comments, [:post_id])   end end 

model:

def changeset(struct, params \\ %{})   struct   |> cast(params, [:post_id])   |> validate_required([:post_id])   |> foreign_key_constraint(:post_id, name: :comments_post_id_fkey) end 

demo:

iex(1)> comment.changeset(%comment{}, %{}).errors [post_id: {"can't blank", []}] iex(2)> comment.changeset(%comment{}, %{post_id: 999}).errors [] iex(3)> {:error, changeset} = comment.changeset(%comment{}, %{post_id: 999}) |> repo.insert; changeset.errors [post_id: {"does not exist", []}] iex(4)> comment.changeset(%comment{}, %{post_id: 1}) |> repo.insert {:ok,  %myapp.comment{__meta__: #ecto.schema.metadata<:loaded, "comments">,   id: 1, inserted_at: #ecto.datetime<2016-07-29 06:25:24>,   post: #ecto.association.notloaded<association :post not loaded>,   post_id: 1, updated_at: #ecto.datetime<2016-07-29 06:25:24>}}