fix: return txid as binary to prevent JSON serialization issues

Author: magnetised

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Changed
+
+- Changed type of returned txid to binary to prevent JSON serialization issues ([#71](https://github.com/electric-sql/phoenix_sync/pull/71))
+
 ## [0.5.0] - 2025-08-13
 
 ### Added

lib/phoenix/sync/writer.ex

@@ -1413,7 +1413,7 @@ if Code.ensure_loaded?(Ecto) do
     end
 
     @txid_name {:__phoenix_sync__, :txid}
-    @txid_query "SELECT txid_current() as txid"
+    @txid_query "SELECT txid_current()::text as txid"
 
     defp start_multi do
       Ecto.Multi.new()

test/phoenix/sync/writer_test.exs

@@ -134,6 +134,10 @@ defmodule Phoenix.Sync.WriterTest do
     end
   end
 
+  defp is_txid(txid) do
+    is_binary(txid) && match?({_, ""}, Integer.parse(txid))
+  end
+
   # new creates without applying (so doesn't need a repo)
   # apply creates and applies i.e. new() |> Repo.transaction()
   describe "allow/2" do
@@ -291,7 +295,7 @@ defmodule Phoenix.Sync.WriterTest do
       assert %Ecto.Multi{} = multi = Writer.to_multi(writer)
       assert {:ok, txid, _values} = Writer.transaction(multi, Repo)
 
-      assert is_integer(txid)
+      assert is_txid(txid)
 
       # validate that the callbacks are being called
 
@@ -892,7 +896,7 @@ defmodule Phoenix.Sync.WriterTest do
 
       assert {:ok, txid} = Writer.txid(changes)
 
-      assert is_integer(txid)
+      assert is_txid(txid)
 
       assert txid == Writer.txid!(changes)
     end
@@ -962,7 +966,7 @@ defmodule Phoenix.Sync.WriterTest do
                  format: Writer.Format.TanstackDB
                )
 
-      assert is_integer(txid)
+      assert is_txid(txid)
       assert_receive {:insert, ["public", "todos_local"], %{}, %{"id" => "98"}}
       assert_receive {:insert, ["public", "todos_local"], %{}, %{"id" => "99"}}
       assert_receive {:delete, ["public", "todos_local"], %{"id" => "2"}, %{}}
@@ -1003,7 +1007,7 @@ defmodule Phoenix.Sync.WriterTest do
                %Support.Todo{id: 100, title: _, completed: false}
              ] = Repo.all(from(t in Support.Todo, order_by: t.id))
 
-      assert is_integer(txid)
+      assert is_txid(txid)
     end
 
     test ":error tuples rollback the transaction" do