[RFC ember-data] modelFactoryFor

text/0000-ember-data-model-factory-for.md

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+- Start Date: 2018-09-06
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# ember-data | modelFactoryFor
+
+## Summary
+
+Promote the private `store._modelFactoryFor` to public API as `store.modelFactoryFor`.
+
+## Motivation
+
+This RFC is a follow-up RFC for [#293 RecordData](https://github.com/emberjs/rfcs/pull/293).
+
+Ember differentiates between `klass` and `factory` for classes registered with the container.
+ At times, `ember-data` needs the `klass`, at other times, it needs the `factory`. For this reason,
+`ember-data` has carried two APIs for accessing one or the other for some time. The public `modelFor`
+ provides access to the `klass` where schema information is stored, while the private `_modelFactoryFor`
+ provides access to the factory for instantiation.
+
+We provide access to the class with `modelFor` roughly implemented as `store._modelFactoryFor(modelName).klass`.
+We instantiate records from this class roughly implemented as `store._modelFactoryFor(modelName).create({ ...args })`.
+
+For symmetry, both of these APIs should be public. Making `modelFactoryFor` public would provide a hook
+ that consumers can override should they desire to provide a custom `ModelClass` as an alternative
+ to `DS.Model`.
+
+## Detailed design
+Due to previous complexity in the lookup of models in `ember-data`, we previously had both `modelFactoryFor`
+and `_modelFactoryFor`. Despite the naming, both of these methods were private. During a recent cleanup phase,
+we unified the methods into `_modelFactoryFor` and left a deprecation in `modelFactoryFor`. This RFC proposes
+un-deprecating the `modelFactoryFor` method and making it public, while deprecating the private `_modelFactoryFor`.
+
+More precisely:
+
+- `store._modelFactoryFor` becomes deprecated and calls `store.modelFactoryFor`.
+- `store.modelFactoryFor` becomes un-deprecated.
+
+### The contract for `modelFactoryFor`
+
+The return value of `modelFactoryFor` MUST be the result of a call to [`applicationInstance.factoryFor`](https://www.emberjs.com/api/ember/3.4/classes/ApplicationInstance/methods/factoryFor?anchor=factoryFor)
+ where `applicationInstance` is the `owner` returned by using `getOwner(this)` to access the `owner` of the `store` instance.
+
+```typescript
+interface Klass {}
+
+interface Factory {
+  klass: Klass,
+  create(): Klass
+}
+
+interface FactoryMap {
+    [factoryName: string]: Factory
+}
+
+declare function factoryFor<K extends keyof FactoryMap>(factoryName: K): FactoryMap[K];
+
+interface Store {
+  modelFactoryFor(modelName: string): ReturnType<typeof factoryFor>;
+}
+```
+
+Users interested in providing a custom class for their `records` and who override `modelFactoryFor`,
+ would not need to also change `modelFor`, as this would be the `klass` accessible via the `factory`.
+
+Users wishing to extend the behavior of `modelFactoryFor` could do so in the following manner:
+
+**Example 1:**
+
+**services/store.js**
+```js
+import { getOwner } from '@ember/application';
+import Store from 'ember-data/store';
+
+export default Store.extend({
+  modelFactoryFor(modelName) {
+    if (someCustomCondition) {
+      return getOwner(this).factoryFor(someFactoryName);
+    }
+
+    return this._super(modelName);
+  }
+});
+```
+
+ #### `Model.modelName`
+
+ `ember-data` currently sets `modelName` onto the `klass` accessible via the `factory`. For classes that do not
+   inherit from `DS.Model` this would not be done, although end users may do so themselves in their implementations
+   if so desired.
+
+### What is a valid factory?
+
+The default export of a custom ModelClass **MUST** conform to the requirements of `Ember.factoryFor`. The requirements
+ of `factoryFor` are currently underspecified; however, in practice, this means that the default export is an
+ instantiable class with a static `create` method and an instance `destroy` method or that inherits from `EmberObject`
+ (which provides such methods).
+
+**Example 2:**
+
+```javascript
+import { assign } from '@ember/polyfills';
+
+export default class CustomModel {
+  constructor(createArgs) {
+    assign(this, createArgs);
+  }
+  destroy() {
+    // ... do teardown
+  }
+  static create(createArgs) {
+    return new this(createArgs);
+  }
+}
+```
+
+**Example 3:**
+
+```javascript
+import EmberObject from '@ember/object';
+
+export default class CustomModel extends EmberObject {
+  constructor(createArgs) {
+    super(createArgs);
+  }
+}
+```
+
+Custom classes for models should expect their constructor to receive a single argument: an object with *at least*
+ the following.
+
+- A `recordData` instance accessible via `getRecordData` (see below)
+- Any properties passed as the second arg to `createRecord`
+- An `owner` accessible via `Ember.getOwner`
+- Any DI injections
+- any other properties that `Ember` chooses to pass to a class instantiated via `factory.create` (currently none)
+
+### getRecordData
+
+Every `record` (instance of the class returned by `modelFactoryFor`) will have an associated [RecordData](https://github.com/emberjs/rfcs/pull/293)
+ which contains the backing data for the id, type, attributes and relationships of that record.
+
+This backing data can be accessed by using the `getRecordData` util on the `record` (or on the `createArgs` passed to
+ a record). Using `getRecordData` on a `record` is only guaranteed after the record has been instantiated. During
+ instantiation, this call should be made on the `createArgs` object passed into the record.
+
+**Example 4**
+
+```javascript
+import { getRecordData } from 'ember-data';
+
+export default class CustomModel {
+  constructor(createArgs) {
+    // during instantiation, `recordData` is available by calling `getRecordData` on createArgs
+    let recordData = getRecordData(createArgs);
+  }
+  someMethod() {
+    // post instantiation, `recordData` is available by calling `getRecordData` on the instance
+    let recordData = getRecordData(this);
+  }
+  destroy() {
+    // ... do teardown
+  }
+  static create(createArgs) {
+    return new this(createArgs);
+  }
+}
+```
+
+## How we teach this
+
+This API would be intended for addon-authors and power users. It is not expected
+that most apps would implement custom models, much as it is not expected that most
+apps would implement custom `RecordData`. The teaching story would be limited to
+documenting the nature and purpose of `modelFactoryFor`.
+
+## Drawbacks
+
+- Users may try to use the hook to instantiate records on their own. Ultimately, the store
+  should still do the instantiating.
+
+## Alternatives
+
+Users could define models in `models/*.js` that utilize a custom `ModelClass`.
+However, such an API for custom classes would exclude the ability to dynamically
+generate classes.
+
+## Unresolved questions
+
+None