How can I use unique filename generator function with arc_ecto?

I’ve already created my own function to generate unique string name per given directory, using a counter stored in the directory.

Ex : name = MyApp.File.generate_name “uploads/user/avatars”

The generated file name doesn’t contain the file extension.
I tried to override Arc.Definition.filename function in the file generated by Arc.g avatar.
But when when I add new user with an avatar pic, file stored in disk name differs from name stored in column “avatar” of user schema. The first name (in disk) is what I expect from overrided filename function, and the second name (in database user table) is the initial uploaded pic name.

The other problem I’m facing is that each time the Arc.Definition.filename function is called the name is generated again. I try inside the function to guess when to generate a new name but it doesn’t work well. For each new user insertion it’s called twice. Here is my overriding code :

  # Override the persisted filenames:
  def filename(version, {file, _}) do
    # is it a storage request ?
    if Map.has_key?(file, :path) do
      IO.inspect file

Could someone guide me please ?

Finally I’ve found why uploaded files are treated twice. The problem was in my user model changeset. Arc_Ecto provides a changeset function “cast_attachments(changeset, params, opts)” that manages the upload stuff. So I didn’t have to add my file field to the default “cast(params, opts)” function of my model changeset. Now that I add the upload field only to Arc_Ecto cast_attachments function, things go very better.
I still don’t have a solution to persist in DB my own generated filename but I found another solution. Now I’m ussing a UUID from elixir-uuid to save the uploaded pics. I just add a uuid column to my user schema, then check for its existence in the model changeset. If it’s nil, then I set it. ^^

  def changeset(struct, params \\ %{}) do
    |> cast(params, [:name, :slug, :age, :status, :uuid])
    |> check_uuid
    |> cast_attachments(params, [:avatar])
    |> NameSlug.maybe_generate_slug
    |> validate_required([:name, :age, :status])

  defp check_uuid(changeset) do
    if get_field(changeset, :uuid) == nil do
      force_change(changeset, :uuid, UUID.uuid1)


I have been fiddling with getting proper filenames in arc/arc_ecto as well.

The final filename function I use right now:

defmodule MyImage do
  use Arc.Definition
  use Arc.Ecto.Definition

  # functions to create thumbnail versions
  # ...

  def filename(version, {file, scope}) do
    # quick fix to prevent
    file_name = Path.basename(file.file_name, Path.extname(file.file_name)) 

My model contains a field with the type Ecto.UUID, and in my changeset function I make sure this is set before the cast_attachments call by using

def mymodel_changeset(mymodel, attrs) do
  |> Map.update(:uuid, Ecto.UUID.generate, fn val -> val || Ecto.UUID.generate end)
  |> cast_attachments(attrs, [:image])
  # other validations, casts, etc.

Main tip to give to people would be that Ecto already has UUID-generation/handling functionality built in, so using an external library is not required.

Oh, and the approach with using Ecto.Changeset.get_field and force_change is definitely cleaner :smiley: !


Your reply is very instructive for me. I just tested Ecto.UUID and found it perfect for the job. I will drop the external library. Thanks. :smile:

For the filename function I keep it simple like this :

# Override the persisted filenames:
  def filename(version, _) do

I just worked on the storage_dir function to make unique path :

  # Override the storage directory:
  def storage_dir(_, {file, scope}) do

This way for each image file uploaded, we’ll have a folder named with generated uuid, containing all the versions created (thumb, original…). The point is that when I have to delete an entry in the DB, I can simply remove its uuid corresponding folder. Beside that, I found it simpler to write my own image_delete function in the custom uploader module :

#delete all versions of an avatar
  def remove(scope) do
    storage_dir(nil, {scope.avatar, scope}) |> File.rm_rf!

To be honest I have to say that I was a bit confused with the delete function of arc, so I wrote my simple own. ^^

To finish I don’t know if will have any impact on DB performance but while I’m using an uuid field for arc._ecto usage, I’m still using an Ecto default primary key for the same model. I guess a proper way to do things, would be using the uuid field as primary key in the same time ?

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I ended up doing this:

def filename(version, {file, scope}) do
  file_name = Path.basename(file.file_name, Path.extname(file.file_name))

:os.system_time should be always unique if I am not wrong :slight_smile:

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Not ‘always’, but close enough for ‘most’ people not to care. :slight_smile:

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