diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3bdbb9a..cd4a302 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.0.4] - 2026-06-13
+
+### Fixed
+
+- Register `unique_constraint` on the audit log primary key (using the configured table name) inside `changelog_changeset/1`. This prevents `Ecto.ConstraintError` on `audit_log_pkey` (or custom_table_pkey) when `*_and_log/*` or `log/*` perform their internal insert while a transaction is open (including from `update_and_log` inside `Ecto.Multi` or app tx). The violation is now turned into a changeset error (existing call sites log and continue). Closes OPS-4615.
+
 ## [1.0.3] - 2026-05-28
 
 ### Fixed
diff --git a/lib/ecto_trail/ecto_trail.ex b/lib/ecto_trail/ecto_trail.ex
index 74d7075..f1fd69b 100644
--- a/lib/ecto_trail/ecto_trail.ex
+++ b/lib/ecto_trail/ecto_trail.ex
@@ -55,6 +55,7 @@ defmodule EctoTrail do
   @default_max_params 65_000
   @changelog_fields [:actor_id, :resource, :resource_id, :changeset, :change_type]
   @not_loaded_pattern "Ecto.Association.NotLoaded"
+  @table_name Application.compile_env(:ecto_trail, :table_name, "audit_log")
 
   defmacro __using__(_) do
     quote do
@@ -572,6 +573,8 @@ defmodule EctoTrail do
   defp map_custom_ecto_type(value), do: value
 
   defp changelog_changeset(attrs) do
-    Changeset.cast(%Changelog{}, attrs, @changelog_fields)
+    %Changelog{}
+    |> Changeset.cast(attrs, @changelog_fields)
+    |> Changeset.unique_constraint(:id, name: "#{@table_name}_pkey")
   end
 end
diff --git a/mix.exs b/mix.exs
index c895a54..e08c811 100644
--- a/mix.exs
+++ b/mix.exs
@@ -1,7 +1,7 @@
 defmodule EctoTrail.Mixfile do
   use Mix.Project
 
-  @version "1.0.3"
+  @version "1.0.4"
 
   def project do
     [
diff --git a/test/unit/ecto_trail_test.exs b/test/unit/ecto_trail_test.exs
index 1a44768..2377935 100644
--- a/test/unit/ecto_trail_test.exs
+++ b/test/unit/ecto_trail_test.exs
@@ -329,4 +329,35 @@ defmodule EctoTrailTest do
                )
     end
   end
+
+  describe "audit_log pkey unique_constraint handling (prevents Ecto.ConstraintError)" do
+    setup do
+      {:ok, schema} = TestRepo.insert(%Resource{name: "pkey-collision-subject"})
+      {:ok, %{schema: schema}}
+    end
+
+    test "update_and_log succeeds (does not raise) when internal audit log insert hits pkey unique violation",
+         %{
+           schema: schema
+         } do
+      # Seed one committed audit row so we have a real id, then force the sequence to hand out the same id on next insert.
+      {:ok, _seed} = TestRepo.insert_and_log(%Resource{name: "pkey-seed"}, "pkey-seed-actor")
+
+      %{rows: [[used_id]]} =
+        TestRepo.query!("SELECT id FROM audit_log ORDER BY id DESC LIMIT 1")
+
+      # Make the next nextval return the id we just observed → the subsequent insert inside update_and_log will collide on audit_log_pkey.
+      TestRepo.query!("SELECT setval('audit_log_id_seq', $1, true)", [used_id])
+
+      # This used to raise Ecto.ConstraintError because changelog_changeset/1 never declared unique_constraint for the pkey.
+      # With the fix it converts the violation to a changeset error; the existing error path in log_changes swallows it (logged)
+      # and the tx still commits the user operation.
+      result =
+        schema
+        |> Changeset.change(%{name: "post-pkey-collision"})
+        |> TestRepo.update_and_log("pkey-test-actor")
+
+      assert {:ok, %Resource{name: "post-pkey-collision"}} = result
+    end
+  end
 end