In general, we use relational databases in developing applications, where our models will be matched to separate database tables. Each model will be related to one or more other modules. However, in WordPress, we have all the custom post types stored in the posts table. Hence, it's not possible to create relationships between different post types with the existing functionality.
WordPress developers around the world have been conducting discussions to get the post relationship capability built into the core. Even though the response from WordPress core development seems positive, we still don't have it on the core version as of 4.7. You can have a look at this interesting discussion at http://make.wordpress.org/core/2013/07/28/potential-roadmap-for-taxonomy-meta-and-post-relationships/.
Since this is one of the most important aspects of web application development...
