Show previous chapter, next chapter navigation in book page of Drupal 8.

Q: I want to show a navigation to previous chapter & next chapter in my book detail page(node view). I'll explain the scenario: I have a parent book and multiple child pages, each may have children up-to level n. I'm showing the level 1(depth 2) pages in a single view(details page) and the children in same page. Here comes the navigation part. I want to show a navigation to previous and next chapters in the Book details page.

A: After creation of the Book page details page, create a view as per the below pictures

1.Create a View-Give name and select the content type. Check 'Create a Block'.

2. As per the scenario I want to see Level 2 book pages(siblings of current page), so I set Depth is equal to 2 under Filter criteria. Sort criteria set us Depth's Ascending order.

3. Created a contextual filter: Node ID and set its default value from the URL(as we are viewing the node view page)

4. Under more we have to check the exclude option so that the result will exclude the current book page.

5.Save the view. See I created two blocks , one for previous link and one for next link. 

By visiting structure > Block Layout you are able to place the newly created blocks.

 6. Now we have to create a module to apply some query conditions to the view. Simply create a module and paste the below code in your module's .module file and enable it.

<?php use Drupal\views\ViewExecutable; use Drupal\views\Plugin\views\query\QueryPluginBase; /** * To get the next and previous chapters * * @param ViewExecutable $view * @param QueryPluginBase $query * @return void */ function chapter_navigation_views_query_alter(ViewExecutable $view, QueryPluginBase $query) { $node = \Drupal::routeMatch()->getParameter('node'); if ($view->id() == 'chapter_navigation' && $node) { $operator = $view->getDisplay()->display['id'] == 'next_chapter' ? '>=' : '<='; foreach ($query->where as &$condition_group) { array_push($condition_group['conditions'], [ 'field' => 'book.weight '.$operator.' ' . $node->book['weight'], 'value' => [], 'operator' => 'formula', ]); } } }

Show child pages in parent book details view page in Drupal 8

Q: I'm using the Book module to maintain the parent-child relationship between content. I want to see all child pages expanded when I see a parent node details page. How can I achieve it?

I tried with THEMENAME_preprocess_book_tree(), but so far I have not been able to achieve that I wanted.
Is this the right hook to use in my case?

A: I managed to find a solution. I would like to post it here if somebody looking for similar solutions.

Make a view for the parent page with all needed configurations. In my case, I have the main chapter page as the parent, the content type is 'chapter'. And when I see the view in a page I want to see all sub-chapters coming under it. The further steps are followed:

1. Browse to /admin/structure/views and click on "Add view".
2. Enter a view name, Select content type from view settings
3. Check the "Create a block" checkbox only. Then click on "Save & edit".
4. On the right-hand side, click on "Advanced" so that the advanced menu appears.
5. Click on "add" next to "Relationships" to add a relationship.
6. Select "1st parent" (the parent book node) and click on "Apply and configure relationships".
7. Check the "Require this relationship" checkbox and click on "Apply".
8. Click on "add" next to "Contextual filters" to add a contextual filter.
9. Select "ID" (The node ID) and click on "Add and configure contextual filters". Choose relationship to "Book 1st Parent"
10. In the "when the filter value is not available" section, select "Provide default value" and in the "Type" drop-down menu, select "Content ID from URL"
11. click on "Apply" & Save the view
12. Place the just created block in the page by visiting structure > block layout

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