Fix Synonyms case sensitive issue

includes/classes/Feature/Search/Synonyms.php

@@ -388,10 +388,12 @@ public function add_search_synonyms( $mapping, $index ) {
 		$mapping['settings']['analysis']['filter'][ $filter_name ] = $this->get_synonym_filter();
 
 		// Tell the analyzer to use our newly created filter.
-		$mapping['settings']['analysis']['analyzer']['default_search']['filter'] = array_values(
-			array_merge(
-				[ $filter_name ],
-				$mapping['settings']['analysis']['analyzer']['default_search']['filter']
+		$mapping['settings']['analysis']['analyzer']['default_search']['filter'] = $this->maybe_change_filter_position(
+			array_values(
+				array_merge(
+					[ $filter_name ],
+					$mapping['settings']['analysis']['analyzer']['default_search']['filter'],
+				)
 			)
 		);
 
@@ -483,11 +485,13 @@ function( $success, $index ) {
 				$setting['index']['analysis']['filter']['ep_synonyms_filter'] = $filter;
 
 				// Add the analyzer.
-				$setting['index']['analysis']['analyzer']['default_search']['filter'] = array_values(
-					array_unique(
-						array_merge(
-							[ $this->get_synonym_filter_name() ],
-							$filters
+				$setting['index']['analysis']['analyzer']['default_search']['filter'] = $this->maybe_change_filter_position(
+					array_values(
+						array_unique(
+							array_merge(
+								[ $this->get_synonym_filter_name() ],
+								$filters
+							)
 						)
 					)
 				);
@@ -823,4 +827,23 @@ private function update_synonym_post( $content ) {
 			true
 		);
 	}
+
+	/**
+	 * Change the position of the lowercase filter to the beginning of the array.
+	 *
+	 * @since 5.1.0
+	 * @param array $filters Array of filters.
+	 * @return array
+	 */
+	protected function maybe_change_filter_position( array $filters ) : array {
+		$lowercase_filter = array_search( 'lowercase', $filters, true );
+
+		if ( false !== $lowercase_filter ) {
+			unset( $filters[ $lowercase_filter ] );
+			array_unshift( $filters, 'lowercase' );
+		}
+
+		return $filters;
+	}
+
 }

tests/php/features/TestSynonyms.php

@@ -160,4 +160,46 @@ public function test_synonyms_with_spaces() {
 		$this->assertTrue( $query->elasticsearch_success );
 		$this->assertSame( $post_id, $query->posts['0'] );
 	}
+
+	/**
+	 * Tests synonyms are case insensitive
+	 *
+	 * @since 5.1.0
+	 * @group synonyms
+	 */
+	public function test_synonyms_case_insensitive() {
+		$instance = $this->getFeature();
+
+		$this->ep_factory->post->create(
+			[
+				'ID'           => $instance->get_synonym_post_id(),
+				'post_content' => 'hoodie, sweatshirt',
+				'post_type'    => $instance::POST_TYPE_NAME,
+			]
+		);
+
+		$instance->update_synonyms();
+
+		$post_id = $this->ep_factory->post->create( [ 'post_content' => 'sweatshirt' ] );
+
+		ElasticPress\Elasticsearch::factory()->refresh_indices();
+
+		$query = new \WP_Query(
+			[
+				's'      => 'HoOdiE',
+				'fields' => 'ids',
+			]
+		);
+		$this->assertTrue( $query->elasticsearch_success );
+		$this->assertSame( $post_id, $query->posts['0'] );
+
+		$query = new \WP_Query(
+			[
+				's'      => 'HOODIE',
+				'fields' => 'ids',
+			]
+		);
+		$this->assertTrue( $query->elasticsearch_success );
+		$this->assertSame( $post_id, $query->posts['0'] );
+	}
 }