[BrainStorm] how to convert a string into an ActiveModel?

[RomainMazB]

Hi!
Thinking about a feature discussed previously, I'm stuck into a problem that I cannot solve and don't even know if it's doable, so I'm opening a brainstorm discussion to solve this problem.

The subject, how to convert a string into an ActiveModel is related to polymorphic relationship.

What is a polymorphic relationship

For those who are not familiar with this kind of relationship, this relationship is more a way to reduce the amount of tables in the database than a magic relationship.

Say we have a Comment Entity, which is related to both Video Entity and Article Entity.
Instead of having 4 tables:

• news
• news_comments which stores on each row the news_id
• videos
• videos_coments which stores on each row the video_id

We could have only one relationship table comments where the relationship is store on two columns instead of one:

• commentable_id, the id of the related news or video
• commentable_type, the full path of the type of the News or Video ActiveRecord like blog::Models::News::ActiveEntity if the comment is related to a News.

Because we can't have a foreign key on the comments table, it's recommended to have an index on the two columns to decrease the performance-loss.

Where is the problem?

The problem is that in higher-level language, it's easy to convert a string into a instance of something, example in PHP we could have done:

$classPath = "App\Models\News";
$news = new $classPath();

This is a mess because the string could be anything, but this is true. We generally make sure that the path actually exists and it's a instance of ActivrRecord, but the type system remains stuck and so does the IDE.

Now you understand the problem: is it possible in Rust to get an instance of a struct with only a string containing it's full path identifier?

Because I bet that it's not possible due to its type system, the real question is: how do we tackle this?

One way to go would be that the RelationDef of the polymorphic relationship store a HashMap with an identifier as keys, and the related instance constructor as value so we could do make sure that a corresponding key exists in the HashMap and return back the instance.

This is one way but maybe not the better one.

Does anyone have a better idea on how this could be done? Or maybe knows a crate that it implements something similar?

Thanks for your bit of knowledge!

[billy1624]

I think this is possible. What we need here is ON CONDITION supports when constructing joins (#160).

We can find the commentable of news and videos correspondingly

• news's commentable: joining both table with news.id = commetable. commentable_id AND TYPE _NEWS = commetable.commetable_type
• videos's commentable: joining both table with videos.id = commetable. commentable_id AND TYPE _VIDEOS = commetable.commetable_type

So, basically we define three entities in Rust news, videos and commentable.