Update Column.js to support parent

addon/classes/Column.js

@@ -139,6 +139,17 @@ export default class Column extends Ember.Object.extend({
    */
   minResizeWidth: 0,
 
+  /**
+   * The parent column (or group) for this sub-column.
+   * This will only have a value if this column is a sub-column.
+   * Note: this doesn't update if you move this sub-column to another parent after instantiation.
+   *
+   * @property parent
+   * @type Column
+   * @optional
+   */
+  parent: null,
+
   /**
    * An array of sub columns to be grouped together
    * @property subColumns
@@ -273,16 +284,6 @@ export default class Column extends Ember.Object.extend({
     return guidFor(this);
   }).readOnly(),
 
-  /**
-   * A reference to the belonging column group. This will only have
-   * a value if this column is a sub column.
-   *
-   * @property format
-   * @type {Function}
-   * @private
-   */
-  _group: null,
-
   /**
    * True if `hidden` or `responsiveHidden` is true.
    * @property isHidden
@@ -334,7 +335,7 @@ export default class Column extends Ember.Object.extend({
     let { subColumns } = options;
 
     subColumns = emberArray(makeArray(subColumns).map((sc) => new Column(sc)));
-    subColumns.setEach('_group', this);
+    subColumns.setEach('parent', this);
 
     this.set('subColumns', subColumns);
   }

addon/mixins/draggable-column.js

@@ -25,17 +25,26 @@ export default Ember.Mixin.create({
     }
   }).readOnly(),
 
-  dragColumnGroup: computed('column._group', function() {
-    let columnGroup = this.get('column._group');
-    return columnGroup ? columnGroup.get('subColumns') : this.get('table.columns');
+  /**
+   * Array of Columns indicating where the column can be potentially dragged.
+   * If the column is part of a group (has a parent column), this will be all of the columns in that group,
+   * otherwise it's all of the columns in the table.
+   *
+   * @property dragColumnGroup
+   * @type Array
+   * @readonly
+   */
+  dragColumnGroup: computed('column.parent', function() {
+    let parent = this.get('column.parent');
+    return parent ? parent.get('subColumns') : this.get('table.columns');
   }).readOnly(),
 
   isDropTarget: computed(function() {
     let column = this.get('column');
     /*
       A column is a valid drop target only if its in the same group
      */
-    return column.get('droppable') && column.get('_group') === sourceColumn.get('_group');
+    return column.get('droppable') && column.get('dragColumnGroup') === sourceColumn.get('dragColumnGroup');
   }).volatile().readOnly(),
 
   dragStart(e) {

tests/unit/classes/column-test.js

@@ -72,3 +72,14 @@ test('CP - visibleSubColumns', function(assert) {
   col.set('hidden', true);
   assert.equal(col.get('visibleSubColumns.length'), 0);
 });
+
+test('subColumns / parent', function(assert) {
+  let col = new Column({
+    subColumns: [{}]
+  });
+  assert.ok(col);
+  assert.equal(col.subColumns.length, 1);
+
+  assert.equal(col.subColumns[0].get('parent'), col);
+
+});