Add blog post highlighting changes in Relay 15

website/blog/2023-03-30-relay-15.mdx

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+---
+title: Relay v15.0 and VSCode Extension v2.0
+author: The Relay Team
+hide_table_of_contents: false
+---
+
+The Relay team is happy to announce the release of Relay v15 as well as v2 of the Relay VSCode extension. These releases are a long time coming and include lots of new feature, bug fixes and quality of life improvements. While this release is a major version bump and includes a couple breaking changes, few users should be affected and upgrading should be seamless for most use cases. You can find the full list of changes in the [v15 Release Notes](https://github.com/facebook/relay/releases/tag/v15.0.0).
+
+## What's new in Relay 15?
+
+### Relay Resolver improvements
+
+A significant portion of our development effort since our last release has gone into improving [Relay Resolvers](https://relay.dev/docs/guides/relay-resolvers) (a mechanism for exposing derived data in the graph). It is worth noting that Relay Resolvers are still experimental and API changes might occur in the future.
+
+#### Terser docblock tags
+
+We've also created a simplified format of the docblock tags necessary to create a Relay Resolver. In many scenarios you can now use a simplified `ParentType.field_name: ReturnType` syntax to define what new field your Relay Resolver exposes.
+
+Before:
+
+```js
+/**
+ * @RelayResolver
+ * @onType User
+ * @fieldName favorite_page
+ * @rootFragment myRootFragment
+ */
+```
+
+After:
+
+```js
+/**
+ * @RelayResolver User.favorite_page: Page
+ * @rootFragment myRootFragment
+ */
+```
+
+#### Define multiple resolvers per file
+
+Prior to this release we only allowed a single Relay Resolver per file and required the Relay Resolver function to be the default export. In Relay 15 you're now able to define multiple Relay Resolvers per file and use named exports. In order to use this feature you'll need to enable the `use_named_imports_for_relay_resolvers` feature flag in your Relay config.
+
+```js
+/**
+ * @RelayResolver User.favorite_page: Page
+ * @rootFragment favoritePageFragment
+ */
+function usersFavoritePage(){
+  ...
+}
+
+/**
+ * @RelayResolver User.best_friend: Friend
+ * @rootFragment bestFriendFragment
+ */
+function usersBestFriend(){
+  ...
+}
+
+module.exports = {
+  usersFavoritePage,
+  usersBestFriend
+}
+```
+
+### Support `@refetchable` on interfaces
+
+Previously it wasn't possible to add the `@refetchable` directive to fragments on server interface types. That support has now been added and will no longer error in the Relay compiler.
+
+```
+// schema.graphql
+
+interface RefetchableInterfaceFoo @fetchable(field_name: "id") {
+    id: ID!
+}
+
+extend type Query {
+  fetch__RefetchableInterfaceFoo(id: ID!): RefetchableInterfaceFoo
+}
+
+// fragment
+
+fragment RefetchableFragmentFoo on RefetchableInterfaceFoo
+  @refetchable(queryName: "RefetchableFragmentFooQuery") {
+  id
+}
+```
+
+### Persisted query improvements
+
+For users who are using URL-based persisted queries, you can now specify custom headers to send with the request to persist your queries. For example, this can be used to send auth headers to your query persistence URL endpoint.
+
+```js
+persistConfig: {
+  url: 'example.com/persist',
+  headers: {
+    Authorization: 'bearer TOKEN'
+  }
+}
+```
+
+And for users who are using file-based persisted queries a new feature flag, `compact_query_text` was added to remove all whitespace from persisted query output text. This can cause a substantial reduction in your persisted query text file (we've seen an example of reducing a single query text from 84kb to 31kb). This new feature flag can be enabled within your Relay config file.
+
+```js
+persistConfig: {
+  file: 'path/to/file.json',
+  algorithm: 'SHA256'
+},
+featureFlags: {
+  compact_query_text: true
+}
+```
+
+### Typesafe updates now support missing field handlers
+
+One of the few breaking changes in this release, the [missing field handlers](https://relay.dev/docs/guided-tour/reusing-cached-data/filling-in-missing-data) function signature has been changed. The `record` argument to the handler used to recieve a `Record` type (which is an untyped grabbag containing whatever the store has) but we now pass a `ReadOnlyRecordProxy` which is much more typesafe and provides a better interface. Furthermore, we also replaced the `field` argument of type `NormalizationLinkedField` with `CommonLinkedField`, which both a `ReaderLinkedField` and `NormalizationLinkedField` "implement".
+
+### Flow type improvements
+
+Flow users will now get types inferred from `graphql` literals with more Relay APIs. No longer do Flow users need to explicitly type the return value of the `usePreloadedQuery`, `useQueryLoader`, `useRefetchableFragment`, `usePaginationFragment`, and `useBlockingPaginationFragment` API methods.
+
+## What's new in the Relay VSCode Extension v2?
+
+The latest version of the [Relay VSCode Extension](https://marketplace.visualstudio.com/items?itemName=meta.relay) adds support for workspaces which contain multiple Relay configs in various subdirectories. Instead of only supporting a single Relay project at a time, you can now configure the `relay.projects` configuration to point to multiple Relay projects in a single workspace. This allows you to seamlessly switch between multiple projects and still be able to get field autocompletion, go to definition on fragments spreads, etc. And with the `relay.autoStartCompiler` you can have the Relay compiler run in watch mode on all projects in your workspace in parallel.
+
+If you were previously using the `relay.rootDirectory` or `relay.pathToConfig` configuration options, those have been deprecated and you'll want to migrate those to the `relay.projects` configuration instead.
+
+Before:
+
+```json
+// VSCode Settings
+"relay.rootDirectory": "path/to/app",
+"relay.pathToConfig": "config/relay.config.js"
+```
+
+After:
+
+```json
+// VSCode Settings
+"relay.projects": [{
+  "name": "my-app",
+  "rootDirectory": "path/to/app",
+  "pathToConfig": "config/relay.config.js"
+}]
+```
+
+Note that your existing settings will continue to work in this release, but you should migrate to the new settings as they'll be removed in a future release.
+
+## What's next?
+
+TODO..
+
+Happy Querying!