Replace REST API with @wpgraphql with @apollographql client.

learndash-graphql.php

<?php
add_filter( 'register_post_type_args', function( $args, $post_type ) {

if ( 'sfwd-courses' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'course';
	$args['graphql_plural_name'] = 'courses';
}

if ( 'sfwd-lessons' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'lesson';
	$args['graphql_plural_name'] = 'lessons';
}

if ( 'sfwd-quiz' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'quiz';
	$args['graphql_plural_name'] = 'quizs';
}
if ( 'sfwd-topic' === $post_type ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'topic';
	$args['graphql_plural_name'] = 'topics';
}

return $args;

}, 10, 2 );


add_filter( 'register_taxonomy_args', function( $args, $taxonomy ) {

if ( 'doc_tag' === $taxonomy ) {
	$args['show_in_graphql'] = true;
	$args['graphql_single_name'] = 'documentTag';
	$args['graphql_plural_name'] = 'documentTags';
}

return $args;

}, 10, 2 );


add_action( 'graphql_register_types', function() {
register_graphql_field( 'Courses', 'featured_media', [
	 'type' => 'String',
	 'description' => __( 'The featured_media of the course', 'learndash' ),
	 'resolve' => function( $post ) {
		 $featured_media = get_post_meta( $post->ID, 'featured_media', true );
		 return ! empty( $featured_media ) ? $featured_media : 'null';
	 }
] );
} );


add_action( 'graphql_register_types', function() {

$post_types = WPGraphQL::$allowed_post_types;

if ( ! empty( $post_types ) && is_array( $post_types ) ) {
	foreach ( $post_types as $post_type ) {
		$post_type_object = get_post_type_object( $post_type );

		if ($post_type === 'sfwd-courses') {
			register_graphql_field( $post_type_object->graphql_single_name, 'blurb', [
				'type' => 'String',
				'description' => __( 'The blurb of the course', 'sfwd-courses' ),
				'resolve' => function( $post ) {
					$blurb = get_post_meta( $post->ID, 'blurb', true );
					return ! empty( $blurb ) ? $blurb : '';
				}
			]);
		}

		if ($post_type === 'sfwd-quiz') {
			register_graphql_field( $post_type_object->graphql_single_name, 'total', [
				'type' => 'String',
				'description' => __( 'The total of the quiz', 'sfwd-quiz' ),
				'resolve' => function( $post ) {
					$price = get_post_meta( $post->ID, 'total', true );
					return ! empty( $price ) ? $price : '';
				}
			]);
		}
	}
}
});

Yes. Once you have enabled wpGraphql and activated the Learndash (single and plural names)

all data and content can be accessible via graphql

Yes. Once you have enabled wpGraphql and activated the Learndash (single and plural names)

all data and content can be accessible via graphql

The issue I am seeing is that when i register the post types using the code above, it will not fetch related lessons of a course via:

{
...lessons
}

Querying for courses will indeed give me back all courses, but not the attached lessons..

Does more code then above need to be added to make that functionality work?

I tested this on a new template site with just learndash and graphql installed.

I had to add this code below the code above to get all my associated topics of a lesson, and lessons to a course:

add_action( 'graphql_register_types', function() {

    register_graphql_connection( [
        'fromType' => 'Course',
        'toType' => 'Lesson',
        'fromFieldName' => 'siblings',
        'resolve' => function( $page, $args, $context, $info ) {
            $courseId = $page->databaseId;
            
            if ( ! $courseId ) {
                return null;
            }

            $resolver = new \WPGraphQL\Data\Connection\PostObjectConnectionResolver( $page, $args, $context, $info );
            $resolver->set_query_arg( 'post_type', 'sfwd-lessons' ); // Assuming 'sfwd-lessons' is your Lesson post type
            $resolver->set_query_arg( 'meta_query', array(
                array(
                    'key' => 'course_id',
                    'value' => $courseId,
                    'compare' => '=',
                ),
            ));

            return $resolver->get_connection();
        }
    ]);
	
	register_graphql_connection( [
        'fromType' => 'Lesson',
        'toType' => 'Topic',
        'fromFieldName' => 'siblings',
        'resolve' => function( $page, $args, $context, $info ) {
            $lessonId = $page->databaseId;
            
            if ( ! $lessonId ) {
                return null;
            }

            $resolver = new \WPGraphQL\Data\Connection\PostObjectConnectionResolver( $page, $args, $context, $info );
			$resolver->set_query_arg( 'post_type', 'sfwd-topic' ); // Assuming 'sfwd-topic' is your Lesson post type
      
            $resolver->set_query_arg( 'meta_query', array(
                array(
                    'key' => 'lesson_id',
                    'value' => $lessonId,
                    'compare' => '=',
                ),
            ));

            return $resolver->get_connection();
        }
    ]);

});

query CoursesAndLessons {
  courses {
    nodes {
      title
      siblings {
        nodes {
          title
          siblings {
            nodes {
              title
            }
          }
        }
      }
    }
  }
}

returns:

{
  "data": {
    "courses": {
      "nodes": [
        {
          "title": "Course 2",
          "siblings": {
            "nodes": [
              {
                "title": "C2 – Lesson 2",
                "siblings": {
                  "nodes": [
                    {
                      "title": "C2 – L2- T2"
                    },
                    {
                      "title": "C2 – L2- T1"
                    }
                  ]
                }
              },
              {
                "title": "C2 – Lesson 1",
                "siblings": {
                  "nodes": []
                }
              }
            ]
          }
        },
        {
          "title": "Course 1",
          "siblings": {
            "nodes": [
              {
                "title": "Lesson 3",
                "siblings": {
                  "nodes": []
                }
              },
              {
                "title": "Lesson 2",
                "siblings": {
                  "nodes": []
                }
              },
              {
                "title": "Lesson 1",
                "siblings": {
                  "nodes": []
                }
              }
            ]
          }
        }
      ]
    }
  }
}