add basic 3 models: node, folder, document

app/models/document.js

-import Model, { attr } from '@ember-data/model';
+import { attr } from '@ember-data/model';
+import NodeModel from "./node";
 
-export default class DocumentModel extends Model {
-  @attr title;
+
+export default class DocumentModel extends NodeModel {
   @attr image;
+
+  get type() {
+    return 'document';
+  }
 }
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app/models/folder.js

+import { attr } from '@ember-data/model';
+import NodeModel from "./node";
+
+
+export default class FolderModel extends NodeModel {
+
+  get type() {
+    return 'folder';
+  }
+
+}
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app/models/node.js

+import Model, { attr } from '@ember-data/model';
+
+
+export default class NodeModel extends Model {
+  @attr title;
+  @attr parent;
+  @attr type;
+}
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