Focus on Joomla's Categories

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<div class="atitle2">Focus on Joomla's 
 <div class="atitle" style="font-size:350% !important">Categories</div>
 
 by Peter Martin / <a href="https://db8.nl" target="_blank">db8.nl</a>
 
 slides: <a href="https://petermartin.nl" target="_blank">https://petermartin.nl</a>
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### Focus on Categories in Joomla

> The information architecture engine behind Joomla: from "what is a category" to ACL, routing, and developer hooks.

---

<div class="sheet">1. The Basics</div>

----

### 1.1 What is a Category?

**Category** = container to group and organise content.

- A smart folder for items.
- Categories help editors organise content.
- And help visitors browse related content.

----

### 1.2 Categories are everywhere

Core Joomla components

- Articles (`com_content`)
- Contacts (`com_contact`)
- Newsfeeds (`com_newsfeeds`)
- Banners (`com_banners`)

Third-party extensions

----

### 1.3 Why use Categories?

| Purpose | Example |
|----------------------------------------|----|
| Organisation | Group "News" articles separately from "Events" |
| Navigation | Build menu items like "Category Blog" or "Category List" |
| Access Control | Restrict a category to "Registered users" only |
| Styling | Apply different layouts or templates per category |
| URL structure | `/news/joomla-6-released` instead of `/article/42` |

----

### 1.4 Where do I find Categories?

In the Joomla 6 backend:

```
System → Manage → Categories (global view)
Content → Articles → Categories (article categories)
Components → Contacts → Categories (contact categories)
Components → Banners → Categories (banner categories)
```

---

<div class="sheet">2. Creating and Managing Categories</div>

----

### 2.1 Creating a Category

`Content → Articles → Categories → New`

Required:

- Title — the human-readable name
- Alias — used in the URL

----

### 2.2 The Category Edit Screen — Tabs

| Tab | What it controls |
|------------------------------------|------------------|
| Category | Title, parent, status, access, language, description |
| Options | Layout overrides, alternative layout, image |
| Publishing | Created/modified dates, author, meta info, version note |
| Permissions | Per-group ACL (create, delete, edit, edit state, edit own) |

----

### 2.3 Key Fields Explained

- Parent — No parent = root category, or other category for nesting.
- Status — Published, Unpublished, Archived, Trashed.
- Access — Public, Guest, Registered, Special, Super Users + custom.
- Language — bind to a specific site language or leave as "All".
- Note — internal-only label, not shown on the frontend.
- Version Note — message stored in the version history.

---

<div class="sheet">3. Nested Categories</div>

----

### 3.1 The Tree Structure

Joomla uses a Nested Set Model to store categories: 
every category has lft and rgt values. 
(makes deeply nested queries fast).

```
ROOT
├── News
│   ├── National
│   └── International
├── Events
│   ├── Workshops
│   └── Meetups
└── About
```

----

### 3.2 How lft and rgt actually work

Take this tree:

```
News
 ├── Company News
 └── Joomla News
      ├── Extensions
      └── Templates
```

Stored as:

```
News           lft=1   rgt=10
Company News    lft=2   rgt=3
Joomla News     lft=4   rgt=9
Extensions      lft=5   rgt=6
Templates       lft=7   rgt=8
```

**A parent always wraps its children.**

----

### 3.3 Reading the tree

A category is **inside** another when:

```sql
child.lft > parent.lft
AND child.rgt < parent.rgt
```

Extensions (5, 6) is inside Joomla News (4, 9) because:

```
5 > 4
6 < 9
```

----

### 3.4 Why this matters

To get all child categories of Joomla News, one query:

```sql
SELECT *
 FROM #__categories
 WHERE lft > 4 AND rgt < 9
 ORDER BY lft;
```

→ Extensions, Templates

**No recursion needed.** Fast reads on huge trees.

----

### 3.5 When the tree breaks

lft/rgt values must stay consistent. 
Things that break them:

- Categories edited directly in the database
- Bad migrations
- Custom extensions inserting categories incorrectly
- Parent-child changes without updating nested set
- Failed saves leaving gaps or duplicate values

Symptoms: wrong ordering, missing children, save errors.

----

### 3.6 The Rebuild button

The Rebuild toolbar button in the Categories fixes:

```
lft, rgt, level, path
```

Use when:

- Ordering looks wrong
- Categories vanish from expected parents
- After a migration
- After raw SQL changes
- Joomla reports tree errors

Not a substitute for correct extension code! 
Extensions should use Joomla's APIs, not raw SQL inserts.

----

### 3.7 Practical rules

- Child category does not automatically inherit articles from its parent.
- Permissions are used from parent (unless overridden).
- Moving a category moves all its children with it.
- Keep depth shallow (2–3 levels). Use Tags for cross-cutting concerns.

---

<div class="sheet">4. Real-world examples</div>

----

### 4.1 News website

```
News
 ├── Politics
 ├── Sports
 ├── Tech
 └── Culture
```

----

### 4.2 University website

```
Departments
 ├── Engineering
 ├── Medicine
 └── Law
```

----

### 4.3 Webshop

```
Products
 ├── Laptops
 ├── Phones
 └── Accessories
```

---

<div class="sheet">5. Categories on the Frontend</div>

----

### 5.1 Menu Item Types

| Menu Item Type | What it shows |
|----|----|
| Category Blog | Articles from a category, in blog layout |
| Category List | Articles from a category, as a sortable, paginated table |
| List All Categories | Tree with all categories from a chosen root |
| Single Article | One article (related, not strictly a category view) |

----

### 5.2 Layout Options (Category Blog)

- Leading articles: full width
- Intro articles: columns (1–6)
- Links: title only
- Pagination, ordering, filters per category

----

### 5.3 URL & Routing

With **SEF URLs enabled**:

```
/news  → category root
/news/national  → child category
/news/national/election-2026  → article inside child
```

Joomla 6's router creates clean nested category paths if aliases are unique.

---

<div class="sheet">6. Access Control (ACL)</div>

----

### 6.1 Two distinct concepts

| Concept | Question it answers |
|----|----|
| Access (Viewing Access Level) | Who can see this? |
| Permissions (Actions) | Who can do what with this? |

----

### 6.2 Access Levels

Set on the Category tab. 
The category + its articles are only visible 
to users in the matching access level.

Built-in levels: 
Public, Guest, Registered, Special, Super Users. 
You can define your own.

----

### 6.3 Permissions (per User Group)

On the Permissions tab:

| Action | Meaning |
|----|----|
| Create | Create new items in this category |
| Delete | Delete items in this category |
| Edit | Edit any item |
| Edit State | Publish, unpublish, archive, trash |
| Edit Own | Edit only items the user created |
| Edit Custom Field Value | Change custom field values |

----

### 6.4 The inheritance chain

```
Global Configuration
   └── Component (e.g. com_content)
         └── Category
               └── Article
```

Each level can be:

- **Inherit** (default)
- **Allowed** (grant)
- **Denied** (hard block, can't be overridden)

> **Deny at the highest level you can; allow at the lowest level you must.**

----

### 6.5 Real-world ACL examples

- Client portals
- Staff-only documentation
- Paid memberships
- Department-only intranet sections

---

<div class="sheet">7. Advanced Features</div>

----

### 7.1 Custom Fields per Category

Content → Fields → Field Groups, then Fields

- Assign fields to **specific categories** so editors only see relevant inputs.
- Repeating data structures? Combine with **subform**.
- Render with `{field N}` shortcodes or in template overrides.

----

### 7.2 Layout Overrides per Category

In your template:

```
templates/your_template/html/com_content/category/
   blog.php
   blog_item.php
   blog_links.php
```

For a single category, use an **Alternative Layout** on the Options tab, or name files with the category alias for finer control.

----

### 7.3 Module Assignment by Category

Modules can be assigned to menu items 
and therefore 
shown per category through menu items linked to category views.

----

### 7.4 Workflows (com_content)

Workflows can be **enabled per category**: articles move through stages (Draft → Review → Published) with transitions controlled by ACL.

Content → Workflows, then bind a workflow to a category via the article options.

----

### 7.5 Tags vs Categories

| Use a Category when… | Use a Tag when… |
|----|----|
| Item belongs to **one** place | Item relates to **many** topics |
| Hierarchy matters | Flat, cross-cutting labels |
| You need ACL or workflow | Discovery / filtering |
| Routing should reflect it | URL not critical |

> Categories define what content is. 
> Tags describe what content is about.

---

<div class="sheet">8. Under the Hood (Developer View)</div>

----

### 8.1 One database table for all extensions

Joomla stores every category in a single table:

```
jos_categories
 ├── com_content categories
 ├── com_contact categories
 ├── com_newsfeeds categories
 └── com_example categories
```

The `extension` column is the filter.

----

### 8.2 com_categories in any extension

Any extension can use Joomla's category manager:

```
/administrator/index.php?option=com_categories&view=categories&extension=com_messages
```

The important part:

```
extension=com_messages
```

→ "Show categories that belong to this extension."

No need to build your own category manager!

----

### 8.3 What you get for free

Using com_categories gives you:

- Nested category support
- ACL integration
- Published / unpublished states
- Metadata fields
- Language support
- Ordering
- Rebuild tooling

----

### 8.4 Important table fields

```
id
parent_id
level
path
extension
lft
rgt
alias
published
access
language
params       (JSON blob with category-specific options)
metadata, metadesc, metakey   (SEO)
```

parent_id, level, lft, rgt, and path describe the tree.

----

### 8.5 Categories API (PHP)

```php
use Joomla\CMS\Categories\Categories;

$categories = Categories::getInstance('Content');
$node       = $categories->get('news');     // by alias or id
$children   = $node->getChildren();
$parent     = $node->getParent();
$articles   = $node->getNumItems();
```

----

### 8.6 Making your component "Category-aware"

For a custom component:

1. Use extension = com_yourcomponent in #__categories.
2. Extend Joomla\CMS\Categories\Categories for your component.
3. Add a rules column to your items table for per-item ACL.
4. Register a router that handles category segments.

----

### 8.7 Events to hook into

| Event | When it fires |
|----|----|
| onCategoryChangeState | Publish / unpublish |
| onContentAfterSave (context com_categories.category) | After save |
| onContentBeforeDelete | Before deletion |

---

<div class="sheet">9. Common Pitfalls</div>

----

### 9.1 Too many top-level categories

Bad:

```
News
Blog
Articles
Updates
Stories
```

Better:

```
Content
 ├── News
 ├── Blog
 └── Updates
```

----

### 9.2 Using categories like tags

Categories should be:

- Strategic
- Stable
- Structural

Do NOT use "one-off" categories. 
Use **Tags** instead.

----

### 9.3 Bad planning

Restructuring later causes:

- Broken URLs
- SEO regressions
- Navigation confusion
- Permission tangles

> Plan categories before building content.

----

### 9.4 Other gotchas

- Forgetting to rebuild after manual database edits → broken lft/rgt.
- Mixing languages a nl-NL category won't show en-GB items.
- Permission confusion component-level "Denied" cannot be overridden.
- Deleting a parent reassigns children to the grandparent, not deletes them.
- Alias collisions under different parents are allowed but can confuse SEF if misconfigured.

---

<div class="sheet">10. Quick Reference Cheat Sheet</div>

----

## Quick Reference Cheat Sheet

```text
CREATE       Content → Articles → Categories → New
NEST         Set "Parent" field
PUBLISH      Status: Published + Access: Public
RESTRICT     Access: Registered (or custom level)
HIERARCHY    Use Rebuild if tree breaks
WORKFLOW     Bind on the Options tab
FIELDS       Assign Custom Fields to specific categories
OVERRIDE     templates/{tpl}/html/com_content/category/
ROUTING      Enable SEF, use unique aliases
ACL          Component → Category → Article (inherited)
REUSE        index.php?option=com_categories&extension=com_X
```

---

----

## 11. Live Demo

1. Create nested categories
2. Assign articles to a category
3. Build a Category Blog menu item
4. Open `com_categories` with `extension=com_messages`
5. Inspect `#__categories` in the database
6. Walk through `lft` / `rgt` on a real row
7. Run **Rebuild** after touching the DB
8. Restrict a category by Access Level
9. Show a SEF-friendly category URL
10. Add a Custom Field bound to one category

---

<div class="sheet">12. Summary</div>

----

## Summary

> In Joomla, categories are not just containers for content.

- Structure: your information architecture
- Navigation: the menus visitors use
- Permissions: who sees and edits what
- SEO: URL paths and topic clustering
- Scalability: growth without rewrites
- Developer integration: a shared API for every extension
- User experience: for both editors and visitors

---

<div class="sheet">Questions?</div>