--- /dev/null
+<?php
+
+namespace Drupal\entity_reference_revisions\Tests;
+
+use Drupal\field_ui\Tests\FieldUiTestTrait;
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests the entity_reference_revisions diff plugin.
+ *
+ * @group entity_reference_revisions
+ *
+ * @dependencies diff
+ */
+class EntityReferenceRevisionsDiffTest extends WebTestBase {
+
+ use FieldUiTestTrait;
+ use EntityReferenceRevisionsCoreVersionUiTestTrait;
+
+ /**
+ * Modules to enable.
+ *
+ * @var array
+ */
+ public static $modules = [
+ 'block_content',
+ 'node',
+ 'field',
+ 'entity_reference_revisions',
+ 'field_ui',
+ 'diff',
+ ];
+
+ /**
+ * {@inheritdoc}
+ */
+ protected function setUp() {
+ parent::setUp();
+ // Create article content type.
+ $this->drupalCreateContentType(['type' => 'article', 'name' => 'Article']);
+
+ // Disable visual inline diff.
+ $config = $this->config('diff.settings')
+ ->set('general_settings.layout_plugins.visual_inline.enabled', FALSE);
+ $config->save();
+
+ $admin_user = $this->drupalCreateUser([
+ 'administer site configuration',
+ 'administer nodes',
+ 'administer content types',
+ 'administer node fields',
+ 'administer node display',
+ 'administer node form display',
+ 'view all revisions',
+ 'edit any article content',
+ ]);
+ $this->drupalLogin($admin_user);
+ $this->drupalPlaceBlock('system_breadcrumb_block');
+ }
+
+ /**
+ * Test for diff plugin of ERR.
+ *
+ * Tests that the diff is displayed when changes are made in an ERR field.
+ */
+ public function testEntityReferenceRevisionsDiff() {
+ // Add an entity_reference_revisions field.
+ static::fieldUIAddNewField('admin/structure/types/manage/article', 'err_field', 'err_field', 'entity_reference_revisions', [
+ 'settings[target_type]' => 'node',
+ 'cardinality' => '-1',
+ ], [
+ 'settings[handler_settings][target_bundles][article]' => TRUE,
+ ]);
+
+ // Create first referenced node.
+ $title_node_1 = 'referenced_node_1';
+ $edit = [
+ 'title[0][value]' => $title_node_1,
+ 'body[0][value]' => 'body_node_1',
+ ];
+ $this->drupalPostNodeForm('node/add/article', $edit, t('Save and publish'));
+
+ // Create second referenced node.
+ $title_node_2 = 'referenced_node_2';
+ $edit = [
+ 'title[0][value]' => $title_node_2,
+ 'body[0][value]' => 'body_node_2',
+ ];
+ $this->drupalPostNodeForm('node/add/article', $edit, t('Save and publish'));
+
+ // Create referencing node.
+ $title = 'referencing_node';
+ $node = $this->drupalGetNodeByTitle($title_node_1);
+ $edit = [
+ 'title[0][value]' => $title,
+ 'field_err_field[0][target_id]' => $title_node_1 . ' (' . $node->id() . ')',
+ ];
+ $this->drupalPostNodeForm('node/add/article', $edit, t('Save and publish'));
+
+ // Check the plugin is set.
+ $this->drupalGet('admin/config/content/diff/fields');
+ $this->drupalPostForm(NULL, ['fields[node.field_err_field][plugin][type]' => 'entity_reference_revisions_field_diff_builder'], t('Save'));
+
+ // Update the referenced node of the err field and create a new revision.
+ $node = $this->drupalGetNodeByTitle($title);
+ $referenced_node_new = $this->drupalGetNodeByTitle($title_node_2);
+ $edit = [
+ 'field_err_field[0][target_id]' => $title_node_2 . ' (' . $referenced_node_new->id() . ')',
+ 'revision' => TRUE,
+ ];
+ $this->drupalPostNodeForm('node/' . $node->id() . '/edit', $edit, t('Save and keep published'));
+
+ // Compare the revisions of the referencing node.
+ $this->drupalPostForm('node/' . $node->id() . '/revisions', [], t('Compare selected revisions'));
+
+ // Assert the field changes.
+ $this->assertRaw('class="diff-context diff-deletedline">' . $title_node_1);
+ $this->assertRaw('class="diff-context diff-addedline">' . $title_node_2);
+ }
+
+}