EmberData | Polymorphic Relationship Support

text/0793-polymporphic-relations-without-inheritance.md

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+---
+Stage: Accepted
+Start Date: 
+Release Date: Unreleased
+Release Versions:
+  ember-source: vX.Y.Z
+  ember-data: vX.Y.Z
+Relevant Team(s): 
+RFC PR: https://github.com/emberjs/rfcs/pull/793
+---
+
+<!--- 
+Directions for above: 
+
+Stage: Leave as is
+Start Date: 2022-02-11
+Release Date: Leave as is
+Release Versions: Leave as is
+Relevant Team(s): Ember Data
+RFC PR: https://github.com/emberjs/rfcs/pull/793
+-->
+
+# Polymorphic relations without inheritance
+
+## Summary
+
+Allow setting polymorph relations without inheritance and mixins.
+
+## Motivation
+
+In pre-octane Ember you could set polymorphic relations using two ways: inheritance or mixins. Mixins are deprecated and impossible to use with native classes, so the only option is to use inheritance:
+
+```js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class TagModel extends Model {
+  @belongsTo('taggable', { polymorphic: true }) taggable;
+}
+
+// not a model, just a way to make relations work
+class TaggableModel extends Model {
+  @hasMany('tag', { inverse: 'taggable' }) tags;
+}
+
+// real models with tags
+class CommentModel extends TaggableModel {}
+class PostModel extends TaggableModel {}
+```
+
+Which fells apart when you need your model to be a target of several polymorphic relations.
+
+```js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class ViewModel extends Model {
+  @belongsTo('viewable', { polymorphic: true }) viewable;
+}
+
+class ViewableModel extends Model {
+  @hasMany('views', { inverse: 'viewable' }) views;
+}
+
+// How to make Comment and Post viewable?...
+```
+
+You can do some dirty tricks making it inherit from each other  `ViewableModel extends TaggableModel`. Or even by adding new base model class, like this:
+
+```js
+
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class ModelWithPolymorphic extends Model {}
+
+class TagModel extends Model {
+  @belongsTo('model-with-polymorphic', { polymorphic: true }) taggable;
+}
+
+class ViewModel extends Model {
+  @belongsTo('model-with-polymorphic', { polymorphic: true }) viewable;
+}
+
+class CommentModel extends ModelWithPolymorphic {
+  @hasMany('views', { inverse: 'viewable' }) views;
+  @hasMany('tag', { inverse: 'taggable' }) tags;
+}
+
+```
+
+While it solves the problem, it seems like a misuse of inheritance and too much trickery involved.
+
+## Detailed design
+
+I propose to introduce new API, similar to what [rails have](https://guides.rubyonrails.org/association_basics.html#polymorphic-associations).
+
+```js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class TagModel extends Model {
+  @belongsTo('taggable', { polymorphic: true }) taggable;
+}
+
+class ViewModel extends Model {
+  @belongsTo('viewable', { polymorphic: true }) viewable;
+}
+
+class CommentModel extends Model {
+  @hasMany('views', { inverse: 'viewable' }) views;
+  @hasMany('tag', { inverse: 'taggable' }) tags;
+}
+```
+
+`taggable` and `viewable` in this example should be typed as `extends Model`, so any model can take that spot. 
+JSON:API has enough information (`type` and `id`) to set this relation.
+
+## How we teach this
+
+> Would the acceptance of this proposal mean the Ember guides must be
+re-organized or altered? Does it change how Ember is taught to new users
+at any level?
+
+The docs should be changed to promote the new way.
+
+## Drawbacks
+
+None
+
+## Alternatives
+
+> What other designs have been considered? What is the impact of not doing this?
+
+> This section could also include prior art, that is, how other frameworks in the same domain have solved this problem.
+
+## Questions
+
+It might be considered to use `as` instead of `inverse` to make it stand out as polymorphic relation, but if it works without it, I think it's better to keep `inverse`.