Merge pull request #1410 from SergeAstapov/docs-transform

docs: add Ember Data transform page

Author: chriskrycho

docs/ember-data/transforms.md

@@ -0,0 +1,53 @@
+# Transforms
+
+`DS.attr` defines an attribute on a [DS.Model](https://guides.emberjs.com/release/models/defining-models/).
+By default, attributes are passed through as-is, however you can specify an
+optional type to have the value automatically transformed.
+Ember Data ships with four basic transform types: `string`, `number`, `boolean` and `date`.
+
+You can define your own transforms by subclassing [DS.Transform](https://guides.emberjs.com/release/models/defining-models/#toc_custom-transforms).
+Ember Data transforms are normal TypeScript classes.
+The return type of `deserialize` method becomes type of the model class property.
+
+You may define your own transforms in TypeScript like so:
+```typescript
+# app/transforms/coordinate-point.ts
+import Transform from '@ember-data/serializer/transform';
+
+declare module 'ember-data/types/registries/transform' {
+  export default interface TransformRegistry {
+    'coordinate-point': CoordinatePointTransform;
+  }
+}
+
+export type CoordinatePoint = {
+  x: number;
+  y: number;
+};
+
+export default class CoordinatePointTransform extends Transform {
+  deserialize(serialized): CoordinatePoint {
+    return { x: value[0], y: value[1] };
+  }
+
+  serialize(value): number {
+    return [value.x, value.y];
+  }
+}
+
+# app/models/cursor.ts
+import Model, { attr } from '@ember-data/model';
+import { CoordinatePoint } from 'agwa-data/transforms/coordinate-point';
+
+declare module 'ember-data/types/registries/transform' {
+  export default interface ModelRegistry {
+    cursor: Cursor;
+  }
+}
+
+export default class Cursor extends Model {
+  @attr('coordinate-point') declare position: CoordinatePoint;
+}
+```
+
+Note that you should declare your own transform under `TransformRegistry` to make `attr` to work with your transform.