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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/739
+---
+
+# EmberData | Deprecate Non Strict Relationships
+
+## Summary
+
+Deprecates various shorthands for defining a `belongsTo` or `hasMany` 
+relationship that create ambiguity, cause expensive runtime lookups, 
+or hinder static analysis.
+
+## Motivation
+
+Deprecating these shorthands allows us to provide better tooling, remove the bulky
+and expensive code necessary to determine the outcomes of these shorthands at 
+runtime, and in the process clarify and simplify relationship behaviors in the
+documentation. Further, it allows us to align our own internal usage of 
+"relationship meta" with the API put forward in [RFC#487 Custom Model Classes](https://github.com/emberjs/rfcs/blob/master/text/0487-custom-model-classes.md#exposing-schema-information).
+
+## Detailed design
+
+Three shorthands are proposed to be deprecated, with the deprecation targeting `5.0`
+and being `enabled` no-sooner than `4.1`. It may be `available` sooner.
+
+- Deprecates the relationship shorthand that does not provide the related type as the first argument.
+
+This deprecation is related to deprecating `non-strict` types as it requires us to
+normalize the property name into a type string in a manner which may not be correct.
+
+*Before*
+
+```./before.js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+export default class Comment extends Model {
+  @belongsTo()
+  author;
+
+  @hasMany({ async: true, inverse: null })
+  likes;
+}
+```
+
+*After*
+
+```./after.js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+export default class Comment extends Model {
+  @belongsTo('author')
+  author;
+  @hasMany('like', { async: true, inverse: null })
+  likes;
+}
+```
+
+- Deprecates not explicitly setting the relationship inverse property (or `null`).
+
+The current default when not specified is to attempt to find a property on the
+related model that either explicity or implicitly points back, resolving to `null`
+if no such relationship is found and erring if more than one potential inverse was
+discovered.
+
+*Before*
+
+```./before.js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class Comment extends Model {
+  @belongsTo('user')
+  author;
+
+  @hasMany('like')
+  likes;
+}
+
+class User extends Model {
+  @hasMany('comment')
+  comments;
+}
+
+class Like extends Model {
+  @belongsTo('user')
+  author;
+}
+```
+
+*After*
+
+```./after.js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class Comment extends Model {
+  @belongsTo('user', { inverse: 'comments' })
+  author;
+
+  @hasMany('like', { inverse: null })
+  likes;
+}
+
+class User extends Model {
+  @hasMany('comment', { inverse: 'author' })
+  comments;
+}
+
+class Like extends Model {
+  @belongsTo('user', { inverse: null })
+  author;
+}
+```
+
+- Deprecates not explicitly setting `async` to `true|false`.
+
+The current default when not specified is `true`.
+
+*Before*
+
+```./before.js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class Comment extends Model {
+  @belongsTo('user', { inverse: 'comments' })
+  author;
+
+  @hasMany('like', { inverse: null })
+  likes;
+}
+
+class User extends Model {
+  @hasMany('comment', { inverse: 'author' })
+  comments;
+}
+
+class Like extends Model {
+  @belongsTo('user', { inverse: null })
+  author;
+}
+```
+
+*After*
+
+```./after.js
+import Model, { belongsTo, hasMany } from '@ember-data/model';
+
+class Comment extends Model {
+  @belongsTo('user', { async: true, inverse: 'comments' })
+  author;
+
+  @hasMany('like', { async: true, inverse: null })
+  likes;
+}
+
+class User extends Model {
+  @hasMany('comment', { async: true, inverse: 'author' })
+  comments;
+}
+
+class Like extends Model {
+  @belongsTo('user', { async: true, inverse: null })
+  author;
+}
+```
+
+## Codemod
+
+A codemod should be provided. This codemod would analyze a user's relationships
+and wherever possible format the provided options with the now deprecated missing
+information. Note: For the related type and inverse property name when 
+mixin-based polymorphism is present codemods may not be practical.
+
+## How we teach this
+
+Generally these changes improve our ability to document and explain relationship
+APIs as confusing behaviors (such as undefined `async` defaulting to `true`) and
+inverse determination are no longer magical resolutions but explicit declarations.
+API documentation will be updated and any examples in the guides should be as well.
+  
+## Drawbacks
+
+Some churn, but via codemod we can make this quick and seamless for most apps.
+
+## Alternatives
+
+Provide new decorators that replace these. Leave these alone. While we may introduce
+new decorator primitives in the near future, iterating on the existing decorators to
+make them stricter in largely painless ways will allow more teams to migrate to a full
+replacement in the future with greater ease.