Ensure parent chapter is set on section before initial sort

Chapter positions are needed to sort the global sections
collection. Ensure parent model is already set during initial sort,
such that previous sections can be determined correctly right from the
start.

Otherwise, the outline displays an incorrect transition after
inserting a section since the previous section can not yet be used to
determine available transitions.

REDMINE-20217

package/spec/editor/collections/ForeignKeySubsetCollection-spec.js

@@ -180,6 +180,24 @@ describe('ForeignKeySubsetCollection', () => {
     expect(postComments.last().post).toBe(post);
   });
 
+  it('sets reference before intial sort of parent', () => {
+    const post = new Backbone.Model({id: 5});
+    const comments = new Backbone.Collection([
+    ], {comparator: 'position'});
+    const postComments = new ForeignKeySubsetCollection({
+      parentModel: post,
+      parent: comments,
+      foreignKeyAttribute: 'postId',
+      parentReferenceAttribute: 'post'
+    });
+    let referenceOnInitialSort;
+
+    comments.once('sort', () => referenceOnInitialSort = comments.first().post);
+    postComments.add({id: 2})
+
+    expect(referenceOnInitialSort).toBe(post);
+  });
+
   it('removes reference to parent model when model is removed from collection', () => {
     const post = new Backbone.Model({id: 5});
     const comments = new Backbone.Collection([

package/src/editor/collections/ForeignKeySubsetCollection.js

@@ -27,6 +27,14 @@ export const ForeignKeySubsetCollection = SubsetCollection.extend({
 
     this.autoConsolidatePositions = options.autoConsolidatePositions;
 
+    this.listenTo(this, 'add', function(model) {
+      if (options.parentReferenceAttribute) {
+        model[options.parentReferenceAttribute] = parentModel;
+      }
+
+      model.set(options.foreignKeyAttribute, parentModel.id);
+    });
+
     SubsetCollection.prototype.constructor.call(this, {
       parent,
       parentModel,
@@ -41,14 +49,6 @@ export const ForeignKeySubsetCollection = SubsetCollection.extend({
       }
     });
 
-    this.listenTo(this, 'add', function(model) {
-      if (options.parentReferenceAttribute) {
-        model[options.parentReferenceAttribute] = parentModel;
-      }
-
-      model.set(options.foreignKeyAttribute, parentModel.id);
-    });
-
     this.listenTo(parentModel, 'destroy dependentDestroy', function() {
       this.invoke('trigger', 'dependentDestroy');
       this.clear();