Later: I have realized this applies to validation errors, not constraints, as I’d originally thought. See footnote [3]

I have a changeset with both a constraint and optimistic locking:

    %Animal{id: id}
    |> cast(attrs, [:name, :lock_version])
    |> unique_constraint(:name, name: "unique_available_names")
    |> optimistic_lock(:lock_version)

IF the unique_constraint is violated, I get an {:error, changeset} back, but the changeset contains an incremented version of the :lock_version. Since the update didn’t happen, the version in the changeset will be used to populate the edit form, which uses a hidden input to send back the lock version:

<%= form_for @changeset, ...
  <%= text_input(f, :name) %> ...
  <%= hidden_input(f, :lock_version) %>    <<<<<<<<<<<

A corrected update attempt will now fail with a StaleEntryError.[1]

Am I misunderstanding the right way to use optimistic locking?[2] Details below.

[1] I assume the error check uses equality rather than >=, but I only traced the code to Ecto.Adapters.Sql.schema, which is not far enough.

[2] This seems a different problem than 25570.

[3] I’ve confirmed that this also happens with a validate_length error. That is, the :lock_version is set to 2 in the error changeset, even though the :lock_version in the on-disk copy is 1.

full changeset: #Ecto.Changeset<
  action: :update,
  changes: %{lock_version: 2, name: "preexisting"},   <<<<<<<<<<<<<<
  errors: [
    name: {"should be %{count} character(s)",
     [count: 3838, validation: :length, kind: :is, type: :string]}
  ],
  data: #Crit.Usables.Animal<>,
  valid?: false
>

Start with two animals:

%Crit.Usables.Animal{
  available: true,
  id: 48789,
  lock_version: 1,
  name: "Original Bossie",
}

%Crit.Usables.Animal{
  id: 48790,
  lock_version: 1,
  name: "preexisting",
}

The form will send back these params:

%{"lock_version" => "1", "name" => "preexisting"}

However, the changeset pipeline shown above transforms the lock version before it hits Repo.update:

update: #Ecto.Changeset<
  action: nil,
  changes: %{lock_version: 2, name: "preexisting"},
  errors: [],
  data: #Crit.Usables.Animal<>,
  valid?: true
>

… and the resulting error changeset contains the updated :lock_version:

#Ecto.Changeset<
  action: :update,
  changes: %{lock_version: 2, name: "preexisting"},       <<<<<<<<<<<<<<
  errors: [
    name: {"has already been taken",
     [constraint: :unique, constraint_name: "unique_available_names"]}
  ],
  data: #Crit.Usables.Animal<>,
  valid?: false
>

Might be worth noting that the original Animal (changeset.data) has not been changed.