The plugin threw an array of errors when I installed it, one of them the one mentioned by @alvarofranz but it is not the only - there is also some stuff about non-static methods called statically, and the code itself looks awful in the plugin. It does clearly not follow a tiny bit of the WPCS, for example.
Looking at the code I also see that it is not well organised. For example it creates database tables in the same class as it retrieves data, updates data, deletes data or creates relationships. This is not scalable.
The query aspect is totally inexistent and it is likely one of the biggest aspects of this project (it has a bare naked file but no code).
So, I believe it is not worth to use this plugin as a base - also because it actually seems to me (if I recall right) a more-or-less “cut and copy parts of” Posts 2 Posts by scribu.
And that plugin is old (while not broken, it is also not maintained anymore actively and was written many years ago).
What I am proposing is that we start with a (fresh) plan.
There are several things to be thought of (when Toolset implemented “real” relationships, for example, even thou a study and plan was conducted and prepared for almost 2 years before shipping, and we had years of experience on the subject already and what users do with it, when it was done it turned out several things could have be done better/differently/more complete).
Let’s use that experience.
To start with, we need to define types of relationship, because there is not just 1:1, 1:m and m:m but there is also distinct self relationships (relationships between the same object types, or also known as “references”, and the question of intermediary objects (IO), and how those can be used specially in m:m)
An additional big aspect is localisation, which is better thought of early, even if there is no actual translation feature in CP or its plugins, there might be one day and that needs to be incorporated early). Is a relationship automatically duplicated to its translation? What about objects that are usually not translated like Users? What about eventual IO meta data? etc.
The API aspect additionally, which can hit performance heavily, mostly in terms of retrieving related data, needs to be well thought thru and only knowing how The API should look and work will then define as well how things need to be implemented.
We need to decide where the actual relationship definitions are stored (as a Post Type? As an option? as Metadata?) and how the syntax to define the relationship should look like.
And not to forget (and the current plugin as well as Posts 2 Posts fully neglects that) is the UI both to define the relationships as well as then actually connect objects.
This needs to be straight forward for the end user - allowing to create relationships and then connect objects in an easy to use and highly performant UI (imagine your blog has 10k posts or perhaps 50K. A select with 10k posts to be possibly connected will crash your site, so that needs to be addressed early, with things like an ajax query in a select2 (maybe), where you show just 10 results and then 10 more or by type + search, for example).
Further, there is a powerful aspect of Relationships when using intermediary objects (IO) where you might want to attach meta data (custom fields in general) to that IO.
Then, you have the whole question of how to call related data (like, call a custom field that belongs to the parent of a given post) and how we solve that (either by first query said parent post and then its fields, or by directly providing a syntax that allows to call the parent, such as
get_post_meta( $post->parent ) for example, which would work only in 1:1 or 1:m from the m side, but never in an m:m or from the 1 side in a 1:m because there might be an array of “parents”).
This is a huge project that will reach stalemate in no time if we do not plan it well ahead before even thinking about code, or naming conventions (which is something that of course must be defined early, definitely, but how we name the table will be the very smallest of all issues )
I don’t think the petition here is the right place to discuss and define all that, and I don’t think starting off with code is (at this point in time) adequate.
What is the best tool for something like this? Google Docs?
So we can add comment or edit cap and once we have a well defined project we can start the actual coding process?