# 🍱 Collections
## What are Collections?
Astro Collections help you break up a larger set of data into multiple pages. Examples of use-cases include:
- Pagination: `/posts/1`, `/posts/2`, etc.
- Grouping content by author: `/author/fred`, `/author/matthew`, etc.
- Grouping content by some tag: `/tags/red`, `/tags/blue`, etc.
- Working with remote data
- Mixing remote and local data
**When to use Collections: When you need to reuse a single template to generate multiple pages from a larger dataset.** If you just want to generate a single page (ex: a long list of every post on your site) then you can just fetch that data on a normal Astro page without using the Collection API.
## Collections API
To create a new Astro Collection, you must do three things:
1. Create a new file in the `src/pages` directory that starts with the `$` symbol. This is required to enable the Collections API.
- Example: `src/pages/$posts.astro` -> `/posts/1`, `/posts/2`, etc.
- Example: `src/pages/$tags.astro` -> `/tags/:tag` (or `/tags/:tag/1`)
2. Define and export the `collection` prop: `collection.data` is how you'll access the data for every page in the collection. Astro populates this prop for you automatically. It MUST be named `collection` and it must be exported.
- Example: `export let collection;`
3. Define and export `createCollection` function: this tells Astro how to load and structure your collection data. Check out the examples below for documentation on how it should be implemented. It MUST be named `createCollection` and it must be exported.
- Example: `export async function createCollection() { /* ... */ }`
- API Reference: [createCollection][collection-api]
## Example: Simple Pagination
```jsx
---
// Define the `collection` prop.
export let collection: any;
// Define a `createCollection` function.
export async function createCollection() {
const allPosts = Astro.fetchContent('../posts/*.md'); // fetch local posts.
allPosts.sort((a, b) => a.title.localeCompare(b.title)); // sort by title.
return {
// Because you are not doing anything more than simple pagination,
// its fine to just return the full set of posts for the collection data.
async data() { return allPosts; },
// number of posts loaded per page (default: 25)
pageSize: 10,
};
}
---
<html lang="en">
<head>
<title>Pagination Example: Page Number {collection.page.current}</title>
</head>
<body>
{collection.data.map((post) => (
<h1>{post.title}</h1>
<time>{formatDate(post.published_at)}</time>
<a href={post.url}>Read Post</a>
))}
</body>
</html>
```
## Example: Pagination Metadata
```jsx
---
// In addition to `collection.data` usage illustrated above, the `collection`
// prop also provides some important metadata for you to use, like: `collection.page`,
// `collection.url`, `collection.start`, `collection.end`, and `collection.total`.
// In this example, we'll use these values to do pagination in the template.
export let collection: any;
export async function createCollection() { /* See Previous Example */ }
---
<html lang="en">
<head>
<title>Pagination Example: Page Number {collection.page.current}</title>
<link rel="canonical" href={collection.url.current} />
<link rel="prev" href={collection.url.prev} />
<link rel="next" href={collection.url.next} />
</head>
<body>
<main>
<h5>Results {collection.start + 1}–{collection.end + 1} of {collection.total}</h5>
{collection.data.map((post) => (
<h1>{post.title}</h1>
<time>{formatDate(post.published_at)}</time>
<a href={post.url}>Read Post</a>
))}
</main>
<footer>
<h4>Page {collection.page.current} / {collection.page.last}</h4>
<nav class="nav">
<a class="prev" href={collection.url.prev || '#'}>Prev</a>
<a class="next" href={collection.url.next || '#'}>Next</a>
</nav>
</footer>
</body>
</html>
```
## Example: Grouping Content by Tag, Author, etc.
```jsx
---
// Define the `collection` prop.
export let collection: any;
// Define a `createCollection` function.
// In this example, we'll customize the URLs that we generate to
// create a new page to group every pokemon by first letter of their name.
export async function createCollection() {
const allPokemonResponse = await fetch(`https://pokeapi.co/api/v2/pokemon?limit=150`);
const allPokemonResult = await allPokemonResponse.json();
const allPokemon = allPokemonResult.results;
const allLetters = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'];
return {
// `routes` defines the total collection of routes as `params` data objects.
// In this example, we format each letter (ex: "a") to params (ex: {letter: "a"}).
routes: allLetters.map(letter => {
const params = {letter};
return params;
}),
// `permalink` defines the final URL for each route object defined in `routes`.
// It should always match the file location (ex: `src/pages/$pokemon.astro`).
permalink: ({ params }) => `/pokemon/${params.letter}`,
// `data` is now responsible for return the data for each page.
// Luckily we had already loaded all of the data at the top of the function,
// so we just filter the data here to group pages by first letter.
// If you needed to fetch more data for each page, you can do that here as well.
async data({ params }) {
return allPokemon.filter((pokemon) => pokemon.name[0] === params.letter);
},
// Finally, `pageSize` and `pagination` is still on by default. Because
// we don't want to paginate the already-grouped pages a second time, we'll
// disable pagination.
pageSize: 1,
};
}
---
<html lang="en">
<head>
<title>Pokemon: {collection.params.letter}</head>
<body>
{collection.data.map((pokemon) => (<h1>{pokemon.name}</h1>))}
</body>
</html>
```
## Example: Individual Pages from a Collection
```jsx
---
// Define the `collection` prop.
export let collection: any;
// Define a `createCollection` function.
// In this example, we'll create a new page for every single pokemon.
export async function createCollection() {
const allPokemonResponse = await fetch(`https://pokeapi.co/api/v2/pokemon?limit=150`);
const allPokemonResult = await allPokemonResponse.json();
const allPokemon = allPokemonResult.result;
return {
// `routes` defines the total collection of routes as data objects.
routes: allPokemon.map((pokemon, i) => {
const params = {name: pokemon.name, index: i};
return params;
}),
// `permalink` defines the final URL for each route object defined in `routes`.
permalink: ({ params }) => `/pokemon/${params.name}`,
// `data` is now responsible for return the data for each page.
// Luckily we had already loaded all of the data at the top of the function,
// so we just filter the data here to group pages by first letter.
// If you needed to fetch more data for each page, you can do that here as well.
async data({ params }) {
return allPokemon[params.index];
},
// Note: The default pageSize is fine because technically only one data object
// is ever returned per route. We can set it to "1" in this example for completeness.
pageSize: 1,
};
}
---
<html lang="en">
<head>
<title>Pokemon: {collection.params.name}</head>
<body>
Who's that pokemon? It's {collection.data.name}!
</body>
</html>
```
## Tips
- If you find yourself duplicating markup across many pages and collections, you should probably be using more reusable components.
### 📚 Further Reading
- [Fetching data in Astro][docs-data]
- API Reference: [collection][collection-api]
- API Reference: [createCollection()][create-collection-api]
- API Reference: [Creating an RSS feed][create-collection-api]
[docs-data]: ../README.md#-fetching-data
[collection-api]: ./api.md#collection
[create-collection-api]: ./api.md#createcollection
[example-blog]: ../examples/blog
[fetch-content]: ./api.md#fetchcontent