optimistic lock

[Filip Moens]

Hi all,

I'm trying to use the optimistic lock feature in Phoenix, however it doesn't seem to work as I expected.

The Model (as you can see, I changed use Demo.Web, :model to use Ecto.Schema as I think this is the newer way of doing things - not sure though) :

defmodule Demo.Post do
#  use Demo.Web, :model
  use Ecto.Schema

  schema "posts" do
    field :title, :string
    field :lock_version, :integer, default: 1

    timestamps
  end

  @required_fields ~w(title lock_version)
  @optional_fields ~w()

  def changeset(model, params \\ :empty) do
    model
    |> Ecto.Changeset.cast(params, @required_fields, @optional_fields)
    |> Ecto.Changeset.optimistic_lock(:lock_version)
  end
end

The update method in the controller (as generated by Phoenix running "mix phoenix.gen.html Post posts title:string lock_version:integer":

def update(conn, %{"id" => id, "post" => post_params}) do
  post = Repo.get!(Post, id)
  changeset = Post.changeset(post, post_params)

  IO.inspect changeset

  case Repo.update(changeset) do
    {:ok, post} ->
      conn
      |> put_flash(:info, "Post updated successfully.")
      |> redirect(to: post_path(conn, :show, post))
    {:error, changeset} ->
      render(conn, "edit.html", post: post, changeset: changeset)
  end
end

Now, I open 2 different tabs in my browser to go to the edit page of a specific post (current lock_version: 2).

I modify it in tab 1 and click "Submit". The lock_version is updated to 3.

I modify it in tab 2 and click "Submit". I expected an Ecto.StaleModelError, but that didn't happen. The lock version is updated to 4.

Here is the inspect log of the changeset in the controller:

%Ecto.Changeset{action: nil,

 changes: %{lock_version: 4, title: "post a modified again"}, constraints: [],

 errors: [], filters: %{lock_version: 3},

 model: %Demo.Post{__meta__: #Ecto.Schema.Metadata<:loaded>, id: 5,

  inserted_at: #Ecto.DateTime<2016-04-10T12:21:14Z>, lock_version: 3,

  title: "post a modified", updated_at: #Ecto.DateTime<2016-04-10T12:22:10Z>},

 optional: [], opts: [],

 params: %{"lock_version" => "2", "title" => "post a modified again"},

 prepare: [], repo: nil, required: [:title, :lock_version],

 types: %{id: :id, inserted_at: Ecto.DateTime, lock_version: :integer,

   title: :string, updated_at: Ecto.DateTime}, valid?: true, validations: []}

 

As you can see, the lock_version in params is 2.

The lock_version in changes however is 4. 

I suspect this problem (if it is one) has something to do with the Repo.get! call in the controller.

What am I missing / not understanding / doing wrong?

Filip

[José Valim]

You are right, it is a bug in Ecto. It has been fixed in both v1.1 and master branches.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Filip Moens]

Ok, great José.

Not sure if this related: when creating a Post.

In the model: 

field :lock_version, :integer, default: 1

In the Post controller:

def create(conn, %{"post" => post_params}) do
  changeset = Post.changeset(%Post{}, post_params)

  IO.inspect changeset

  case Repo.insert(changeset) do
    {:ok, _post} ->
      conn
      |> put_flash(:info, "Post created successfully.")
      |> redirect(to: post_path(conn, :index))
    {:error, changeset} ->
      render(conn, "new.html", changeset: changeset)
  end
end

After the create I have a Post with lock_version 2 (where I expected 1)

Changeset log:

[info] POST /posts

[debug] Processing by Demo.PostController.create/2

  Parameters: %{"_csrf_token" => "bg8ddS8eHFpUHWMcJm96AzkpXBUUNgAA7Nd9CL7b9JTnG+/QJN/rfw==", "_utf8" => "✓", "post" => %{"title" => "post x"}}

  Pipelines: [:browser]

%Ecto.Changeset{action: nil, changes: %{lock_version: 2, title: "post x"},

 constraints: [], errors: [], filters: %{lock_version: 1},

 model: %Demo.Post{__meta__: #Ecto.Schema.Metadata<:built>, id: nil,

  inserted_at: nil, lock_version: 1, title: nil, updated_at: nil},

 optional: [:lock_version], opts: [], params: %{"title" => "post x"},

 prepare: [], repo: nil, required: [:title],

 types: %{id: :id, inserted_at: Ecto.DateTime, lock_version: :integer,

   title: :string, updated_at: Ecto.DateTime}, valid?: true, validations: []}

[debug] INSERT INTO "posts" ("inserted_at", "updated_at", "lock_version", "title") VALUES ($1, $2, $3, $4) RETURNING "id" [{{2016, 4, 10}, {14, 3, 28, 0}}, {{2016, 4, 10}, {14, 3, 28, 0}}, 2, "post x"] OK query=0.9ms

If I specify in the Model:

field :lock_version, :integer, default: 0

I end up with a lock_version of 1, which is better.

But, in this case, the meaning of "default: 0" for the lock version in the Model seems "odd".

I would expect a default value for the lock_version always to be used on Create, so default: 0 -> 0 after initial Create, or default: 1 -> 1 after initial Create.

As it currently works, things are kind of confusing.

Filip

[José Valim]

All operations will increase the lock. If you don't want to bump it on create, then don't call the optimistic_lock for the create changeset. :)

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-ecto/51386149-1ba3-4722-b035-df742e9066d6%40googlegroups.com.

[Filip Moens]

Goodness, I'm stepping away from my computer for a few hours now - feeling like an idiot :-)