rfc: ember-data | Deprecate Accessing Static Fields On Model Prior To Lookup

text/0741-ember-data-deprecate-model-static-field-access-without-lookup.md

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+---
+Stage: Accepted
+Start Date: 2021-04-23
+Release Date: Unreleased
+Release Versions:
+  ember-source: vX.Y.Z
+  ember-data: vX.Y.Z
+Relevant Team(s): ember-data
+RFC PR: https://github.com/emberjs/rfcs/pull/741
+---
+
+# EmberData | Deprecate Accessing Static Fields On Model Prior To Lookup
+
+## Summary
+
+Deprecates when a user directly imports a model extending `@ember-data/model` and
+attempts to access one of the static fields containing schema information about
+attributes or relationships.
+
+## Motivation
+
+Schema descriptors on classes extending `@ember-data/model` require walking the prototype
+chain to collect inherited attributes and relationships. This isn't possible until an
+`EmberObject`'s private `proto` method has been invoked.
+
+Externally, we feel accessing schema information in this manner is a bad practice that should
+be avoided. Schema information is exposed at runtime via `store.modelFor` in pre- 
+[custom-model-class](https://github.com/emberjs/rfcs/blob/master/text/0487-custom-model-classes.md#custom-model-class-rfc)
+versions and via the schema definition service and store-wrapper in post- custom-model-class 
+versions (3.28+).
+
+Internally, the EmberData team wishes to explore removing our dependence on `computed` 
+properties and `eachComputedProperty` which make use of invoking `proto` for defining
+schema (`attr|belongsTo|hasMany` currently utilize these APIs to build out the schema information).
+
+## Detailed design
+
+If we detect that an access has been made on a class not provided by a factory (the result of
+calling `modelFor`) we would print a deprecation targeting `5.0` that would become `enabled` 
+no-sooner than `4.1` (although it may be made `available` before then).
+
+Most usages of this pattern occur when a user imports a model for a unit test. In these cases
+the appropriate refactor to look it up via the store like so:
+
+```js
+test('my test', async function(assert) {
+  const UserSchema = this.owner.lookup('service:store').modelFor('user');
+  let { attributes } = UserSchema; // access the map of attributes
+});
+```
+
+Or if defining the model in the test file, first register the model like so:
+
+```js
+test('my test', async function(assert) {
+  class User extends Model {
+    @attr name;
+  }
+  this.owner.register('model:user', User);
+
+  const UserSchema = this.owner.lookup('service:store').modelFor('user');
+  let { attributes } = UserSchema; // access the map of attributes
+});
+```
+
+## How we teach this
+
+Generally this pattern has not been widely observed, though we should make sure that api-docs,
+guides, and blueprints are all updated to show the preferred pattern.
+
+## Drawbacks
+
+Potentially some churn if it turns out that lots of users rely on this sort of access, though
+generally this is just another clear step away from `EmberObject` and it better prepares these
+users for accessing schema on models not built off of `@ember-data/model`.
+
+## Alternatives
+
+Ignore that this pattern exists, which seems risky given our momentum towards custom model 
+classes and potentially a different default model than `@ember-data/model`.