Support both 0 and NULL lock_versions

config/initializers/active_record_locking.rb

@@ -19,12 +19,7 @@ module ActiveRecord
         return 0 if attribute_names.empty?
 
         lock_col = self.class.locking_column
-
         previous_lock_value = send(lock_col).to_i
-
-        # This line is added as a patch
-        previous_lock_value = nil if previous_lock_value == '0' || previous_lock_value == 0
-
         increment_lock
 
         attribute_names += [lock_col]
@@ -35,7 +30,8 @@ module ActiveRecord
 
           affected_rows = relation.where(
             self.class.primary_key => id,
-            lock_col => previous_lock_value
+            # Patched because when `lock_version` is read as `0`, it may actually be `NULL` in the DB.
+            lock_col => previous_lock_value == 0 ? [nil, 0] : previous_lock_value
           ).update_all(
             attributes_for_update(attribute_names).map do |name|
               [name, _read_attribute(name)]

spec/initializers/active_record_locking_spec.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'ActiveRecord locking' do
+  let(:issue) { create(:issue) }
+
+  shared_examples 'locked model' do
+    before do
+      issue.update_column(:lock_version, start_lock_version)
+    end
+
+    it 'can be updated' do
+      issue.update(title: "New title")
+
+      expect(issue.reload.lock_version).to eq(new_lock_version)
+    end
+
+    it 'can be deleted' do
+      expect { issue.destroy }.to change { Issue.count }.by(-1)
+    end
+  end
+
+  context 'when lock_version is NULL' do
+    let(:start_lock_version) { nil }
+    let(:new_lock_version) { 1 }
+
+    it_behaves_like 'locked model'
+  end
+
+  context 'when lock_version is 0' do
+    let(:start_lock_version) { 0 }
+    let(:new_lock_version) { 1 }
+
+    it_behaves_like 'locked model'
+  end
+
+  context 'when lock_version is 1' do
+    let(:start_lock_version) { 1 }
+    let(:new_lock_version) { 2 }
+
+    it_behaves_like 'locked model'
+  end
+end