Proposed solution for embedding custom types into taxonomies

types-into-taxes.diff

diff --git a/lib/class-wp-json-posts.php b/lib/class-wp-json-posts.php
index 941c796..a06ed0f 100644
--- a/lib/class-wp-json-posts.php
+++ b/lib/class-wp-json-posts.php
@@ -519,9 +519,10 @@ class WP_JSON_Posts {
 	 *
 	 * @param string|object $type Type name, or type object (internal use)
 	 * @param boolean $_in_collection Is this in a collection? (internal use)
+	 * @param boolean $_in_taxonomy Is this request being added to a taxonomy record? (internal use)
 	 * @return array Post type data
 	 */
-	public function get_post_type( $type, $_in_collection = false ) {
+	public function get_post_type( $type, $_in_collection = false, $_in_taxonomy = false ) {
 		if ( ! is_object( $type ) )
 			$type = get_post_type_object($type);
 
@@ -542,7 +543,7 @@ class WP_JSON_Posts {
 			),
 		);
 
-		if ( $_in_collection )
+		if ( $_in_collection || $_in_taxonomy )
 			$data['meta']['links']['self'] = json_url( '/posts/types/' . $type->name );
 		else
 			$data['meta']['links']['collection'] = json_url( '/posts/types' );
@@ -554,7 +555,7 @@ class WP_JSON_Posts {
 				$data['meta']['links']['archives'] = json_url( add_query_arg( 'type', $type->name, '/posts' ) );
 		}
 
-		return apply_filters( 'json_post_type_data', $data, $type );
+		return apply_filters( 'json_post_type_data', $data, $type, $_in_taxonomy );
 	}
 
 	/**
@@ -1090,6 +1091,22 @@ class WP_JSON_Posts {
 	}
 
 	/**
+	 * Embed post type data into taxonomy data
+	 *
+	 * @uses self::get_post_type()
+	 * @param array $data Taxonomy data
+	 * @param array $taxonomy Internal taxonomy data
+	 * @return array Filtered data
+	 */
+	public function add_post_type_data( $data, $taxonomy ) {
+		foreach( $taxonomy->object_type as $type ) {
+			$data['types'][ $type ] = $this->get_post_type( $type, false, true );
+		}
+
+		return $data;
+	}
+
+	/**
 	 * Helper method for {@see new_post} and {@see edit_post}, containing shared logic.
 	 *
 	 * @since 3.4.0
diff --git a/lib/class-wp-json-taxonomies.php b/lib/class-wp-json-taxonomies.php
index b05ca9e..cc9b0ac 100644
--- a/lib/class-wp-json-taxonomies.php
+++ b/lib/class-wp-json-taxonomies.php
@@ -134,7 +134,7 @@ class WP_JSON_Taxonomies {
 			),
 		);
 
-		return apply_filters( 'json_prepare_taxonomy', $data );
+		return apply_filters( 'json_prepare_taxonomy', $data, $taxonomy );
 	}
 
 	/**
@@ -142,10 +142,13 @@ class WP_JSON_Taxonomies {
 	 *
 	 * @param array $data Type data
 	 * @param array $post Internal type data
+	 * @param boolean $_in_taxonomy The record being filtered is a taxonomy object (internal use)
 	 * @return array Filtered data
 	 */
-	public function add_taxonomy_data( $data, $type ) {
-		$data['taxonomies'] = $this->get_taxonomies( $type->name );
+	public function add_taxonomy_data( $data, $type, $_in_taxonomy = false ) {
+		if ( ! $_in_taxonomy ) {
+			$data['taxonomies'] = $this->get_taxonomies( $type->name );
+		}
 
 		return $data;
 	}
diff --git a/plugin.php b/plugin.php
index 3571b04..82067b3 100644
--- a/plugin.php
+++ b/plugin.php
@@ -75,6 +75,7 @@ function json_api_default_filters($server) {
 	// Posts
 	$wp_json_posts = new WP_JSON_Posts($server);
 	add_filter( 'json_endpoints', array( $wp_json_posts, 'register_routes' ), 0 );
+	add_filter( 'json_prepare_taxonomy', array( $wp_json_posts, 'add_post_type_data' ), 10, 2 );
 
 	// Users
 	$wp_json_users = new WP_JSON_Users($server);
@@ -97,7 +98,7 @@ function json_api_default_filters($server) {
 	// Posts
 	$wp_json_taxonomies = new WP_JSON_Taxonomies($server);
 	add_filter( 'json_endpoints',      array( $wp_json_taxonomies, 'register_routes' ), 2 );
-	add_filter( 'json_post_type_data', array( $wp_json_taxonomies, 'add_taxonomy_data' ), 10, 2 );
+	add_filter( 'json_post_type_data', array( $wp_json_taxonomies, 'add_taxonomy_data' ), 10, 3 );
 	add_filter( 'json_prepare_post',   array( $wp_json_taxonomies, 'add_term_data' ), 10, 3 );
 }
 add_action( 'wp_json_server_before_serve', 'json_api_default_filters', 10, 1 );
diff --git a/tests/test_json_taxonomies.php b/tests/test_json_taxonomies.php
index 4f8acd9..202d9d5 100644
--- a/tests/test_json_taxonomies.php
+++ b/tests/test_json_taxonomies.php
@@ -126,4 +126,18 @@ class WP_Test_JSON_Taxonomies extends WP_UnitTestCase {
 		$this->assertEquals( $category->description, $data['description'] );
 		$this->assertEquals( $category->count, $data['count'] );
 	}
+
+	public function test_add_taxonomy_data() {
+		// Mock type
+		$type = new stdClass;
+		$type->name = 'post';
+
+		// This record is not a taxonomy record: taxonomies should be embedded
+		$data = $this->endpoint->add_taxonomy_data( array(), $type, false );
+		$this->assertArrayHasKey( 'taxonomies', $data );
+
+		// This record is a taxonomy record: taxonomies should NOT be embedded
+		$data_within_taxonomy = $this->endpoint->add_taxonomy_data( array(), $type, true );
+		$this->assertArrayNotHasKey( 'taxonomies', $data_within_taxonomy );
+	}
 }

This follows what I perceived to be the model for embedding that we use to add taxonomies to types, users to posts, etcetera. Specifically,

• An add_post_type_data method is added to the posts class to augment a Taxonomy data object with that taxonomy's associated post types.
• A hook is added in plugin.php to run that add_post_type_data on the json_prepare_taxonomy filter
• The add_taxonomy_data method (hooked to the json_post_type_data filter) is passed a new parameter indicating whether the post type object in question is being embedded from within a taxonomy object; this prevents circular references where add_post_type_data adds post type objects which each have taxonomy objects added through add_taxonomy_data which in turn again have post type objects embedded.... and so on.

Passing $_in_taxonomy all the way through to add_taxonomy_data feels a little weird, but the only alternative I see is to add a separate filter and that feels even more duplicative.

Added unit test