title: Categories
---
summary: A simple pattern for implementing categories.
---
body:

Because Lektor is based on the idea of mirroring the tree structure from the
file system to the final website it sometimes might not be quite clear how
one could achieve any form of filtering (for instance categories).  However
this is actually very easily achieved through the concept of “child
replacement”.  This will give you a quick introduction to how this feature
can be used to implement categories.

## Basic Setup

In this case we assume you have a bunch of projects that can exist in
different categories and it should be possible to see which projects belong
to which category.

## The Models

So we will end up with four models: `projects.ini`, `project.ini`,
`project-categories.ini` and `project-category.ini`.  Here is how they look:

### `projects.ini`

```ini
[model]
name = Projects
label = Projects
hidden = yes
protected = yes

[children]
model = project
order_by = -date, name
```

### `project.ini`

```ini
[model]
name = Project
label = {{ this.name }}
hidden = yes

[fields.name]
label = Name
type = string

[fields.date]
label = Date
type = date

[fields.description]
label = Description
type = markdown

[fields.categories]
label = Categories
type = checkboxes
source = site.query('/project-categories')
```

The above models should be mostly clear.  What is probably not entirely clear
is the `source` parameter for the categories.  Esentially we instruct the
admin panel to fill the selection for the checkboxes from the child pages
below the `project-categories` folder.  This is where we will maintain the
categories.

### `project-categories.ini`

```ini
[model]
name = Project Categories
label = Project Categories
hidden = yes
protected = yes

[children]
model = project-category
order_by = name
```

### `project-category.ini`

```ini
[model]
name = Project Category
label = {{ this.name }}
hidden = yes

[children]
replaced_with = site.query('/projects').filter(F.categories.contains(this))

[fields.name]
label = Name
type = string
```

So this is where the magic lives.  the `replaced_with` key in the `[children]`
section tells Lektor to ignore the child pages that would normally exist below
the path but to substitute them with another query.  In this case we replace
them with all the projects where the category (`this`) is in the checked
set of categories (`F.categories`).

## Initializing The Folders

Now that we have the models, we need to initialize the folders.  This is
necessary because most models are hidden which means they cannot be selected
from the admin interface.  We need the following things:

### `projects/contents.lr`

```
_model: projects
```

### `project-categories/contents.lr`

```
_model: project-categories
----
_slug: projects/categories
```

Here we also tell the system that we want to move the page from
`project-categories/` to `projects/categories/` which looks nicer.  This
means a category named `Foo` will then end up as `projects/categories/foo/`.

## Creating Categories

Now we can head to the admin panel to create some categories.  Each category
that is created will become available for new projects in the category
selection.

## The Templates

To render out all the pages we probably want to use some macros to reuse
markup that appears on multiple pages.

### `projects.html`

```html+jinja
{% extends "layout.html" %}
{% from "macros/projects.html" import
  render_project_list, render_category_nav %}
{% block title %}Projects{% endblock %}
{% block body %}
  <h1>Projects</h1>
  {{ render_category_nav(active=none) }}
  {{ render_project_list(this.children) }}
{% endblock %}
```

### `project.html`

```html+jinja
{% extends "layout.html" %}
{% block title %}{{ this.name }}{% endblock %}
{% block body %}
  <h1>{{ this.name }}</h1>
  <div class="description">{{ this.description }}</div>
{% endblock %}
```

### `project-categories.html`

```html+jinja
{% extends "layout.html" %}
{% from "macros/projects.html" import render_category_nav %}
{% block title %}Project Categories{% endblock %}
{% block body %}
  <h1>Project Categories</h1>
  {{ render_category_nav(active=none) }}
{% endblock %}
```

### `project-category.html`

```html+jinja
{% extends "layout.html" %}
{% from "macros/projects.html" import render_category_nav %}
{% block title %}Project Category {{ this.name }}{% endblock %}
{% block body %}
  <h1>Project Category {{ this.name }}</h1>
  {{ render_category_nav(active=this._id) }}
  {{ render_project_list(this.children) }}
{% endblock %}
```

### `macros/projects.html`

```html+jinja
{% macro render_category_nav(active=none) %}
  <ul>
  {% for category in site.query('/project-categories') %}
    <li{% if category._id == active %} class="active"{% endif
      %}><a href="{{ category|url }}">{{ category.name }}</a>
  {% endfor %}
  </ul>
{% endmacro %}

{% macro render_project_list(projects) %}
  <ul>
  {% for project in projects %}
    <li><a href="{{ project|url }}">{{ project.name }}</a>
  {% endfor %}
  </ul>
{% endmacro %}
```