astro/.changeset/quick-onions-leave.md

astro
major

Refactors legacy content and data collections to use the Content Layer API glob() loader for better performance and to support backwards compatibility. Also introduces the legacy.collections flag for projects that are unable to update to the new behavior immediately.

⚠️ BREAKING CHANGE FOR LEGACY CONTENT COLLECTIONS ⚠️

By default, collections that use the old types (content or data) and do not define a loader are now implemented under the hood using the Content Layer API's built-in glob() loader, with extra backward-compatibility handling.

In order to achieve backwards compatibility with existing content collections, the following have been implemented:

• a glob loader collection is defined, with patterns that match the previous handling (matches src/content/<collection name>/**/*.md and other content extensions depending on installed integrations, with underscore-prefixed files and folders ignored)
• When used in the runtime, the entries have an ID based on the filename in the same format as legacy collections
• A slug field is added with the same format as before
• A render() method is added to the entry, so they can be called using entry.render()
• getEntryBySlug is supported

In order to achieve backwards compatibility with existing data collections, the following have been implemented:

• a glob loader collection is defined, with patterns that match the previous handling (matches src/content/<collection name>/**/*{.json,.yaml} and other data extensions, with underscore-prefixed files and folders ignored)
• Entries have an ID that is not slugified
• getDataEntryById is supported

While this backwards compatibility implementation is able to emulate most of the features of legacy collections, there are some differences and limitations that may cause breaking changes to existing collections:

• In previous versions of Astro, collections would be generated for all folders in src/content/, even if they were not defined in src/content/config.ts. This behavior is now deprecated, and collections should always be defined in src/content/config.ts. For existing collections, these can just be empty declarations (e.g. const blog = defineCollection({})) and Astro will implicitly define your legacy collection for you in a way that is compatible with the new loading behavior.
• The special layout field is not supported in Markdown collection entries. This property is intended only for standalone page files located in src/pages/ and not likely to be in your collection entries. However, if you were using this property, you must now create dynamic routes that include your page styling.
• Sort order of generated collections is non-deterministic and platform-dependent. This means that if you are calling getCollection(), the order in which entries are returned may be different than before. If you need a specific order, you should sort the collection entries yourself.
• image().refine() is not supported. If you need to validate the properties of an image you will need to do this at runtime in your page or component.
• the key argument of getEntry(collection, key) is typed as string, rather than having types for every entry.

A new legacy configuration flag legacy.collections is added for users that want to keep their current legacy (content and data) collections behavior (available in Astro v2 - v4), or who are not yet ready to update their projects:

// astro.config.mjs
import { defineConfig } from 'astro/config';

export default defineConfig({
  legacy: {
    collections: true
  }
});

When set, no changes to your existing collections are necessary, and the restrictions on storing both new and old collections continue to exist: legacy collections (only) must continue to remain in src/content/, while new collections using a loader from the Content Layer API are forbidden in that folder.