fix(docs): add perspective documentation (#240)

Co-authored-by: Martin Jacobsen <174970+mmgj@users.noreply.github.com>

README.md

@@ -65,6 +65,9 @@ export async function updateDocumentTitle(_id, title) {
     - [UMD](#umd)
   - [Specifying API version](#specifying-api-version)
   - [Performing queries](#performing-queries)
+  - [Using perspectives](#using-perspectives)
+    - [`published`](#published)
+    - [`previewDrafts`](#previewdrafts)
   - [Fetching Content Source Maps](#fetching-content-source-maps)
   - [Listening to queries](#listening-to-queries)
   - [Fetch a single document](#fetch-a-single-document)
@@ -405,6 +408,162 @@ client.fetch(query, params).then((bikes) => {
 
 Perform a query using the given parameters (if any).
 
+### Using perspectives
+
+The `perspective` option can be used to specify special filtering behavior for queries. The default value is `raw`, which means no special filtering is applied, while [`published`](#published) and [`previewDrafts`](#previewdrafts) can be used to optimize for specific use cases.
+
+#### `published`
+
+Useful for when you want to be sure that draft documents are not returned in production. Pairs well with private datasets.
+
+With a dataset that looks like this:
+
+```json
+[
+  {
+    "_type": "author",
+    "_id": "ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "name": "George Martin"
+  },
+  {
+    "_type": "author",
+    "_id": "drafts.ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "name": "George R.R. Martin"
+  },
+  {
+    "_type": "author",
+    "_id": "drafts.f4898efe-92c4-4dc0-9c8c-f7480aef17e2",
+    "name": "Stephen King"
+  }
+]
+```
+
+And a query like this:
+
+```ts
+import {createClient} from '@sanity/client'
+
+const client = createClient({
+  ...config,
+  useCdn: true, // set to `false` to bypass the edge cache
+  perspective: 'published',
+})
+
+const authors = await client.fetch('*[_type == "author"]')
+```
+
+Then `authors` will only contain documents that don't have a `drafts.` prefix in their `_id`, in this case just "George Martin":
+
+```json
+[
+  {
+    "_type": "author",
+    "_id": "ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "name": "George Martin"
+  }
+]
+```
+
+#### `previewDrafts`
+
+Designed to help answer the question "What is our app going to look like after all the draft documents are published?".
+
+Given a dataset like this:
+
+```json
+[
+  {
+    "_type": "author",
+    "_id": "ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "name": "George Martin"
+  },
+  {
+    "_type": "author",
+    "_id": "drafts.ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "name": "George R.R. Martin"
+  },
+  {
+    "_type": "author",
+    "_id": "drafts.f4898efe-92c4-4dc0-9c8c-f7480aef17e2",
+    "name": "Stephen King"
+  },
+  {
+    "_type": "author",
+    "_id": "6b3792d2-a9e8-4c79-9982-c7e89f2d1e75",
+    "name": "Terry Pratchett"
+  }
+]
+```
+
+And a query like this:
+
+```ts
+import {createClient} from '@sanity/client'
+
+const client = createClient({
+  ...config,
+  useCdn: false, // the `previewDrafts` perspective requires this to be `false`
+  perspective: 'previewDrafts',
+})
+
+const authors = await client.fetch('*[_type == "author"]')
+```
+
+Then `authors` will look like this. Note that the result dedupes documents with a preference for the draft version:
+
+```json
+[
+  {
+    "_type": "author",
+    "_id": "ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "_originalId": "drafts.ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "name": "George R.R. Martin"
+  },
+  {
+    "_type": "author",
+    "_id": "f4898efe-92c4-4dc0-9c8c-f7480aef17e2",
+    "_originalId": "drafts.f4898efe-92c4-4dc0-9c8c-f7480aef17e2",
+    "name": "Stephen King"
+  },
+  {
+    "_type": "author",
+    "_id": "6b3792d2-a9e8-4c79-9982-c7e89f2d1e75",
+    "_originalId": "6b3792d2-a9e8-4c79-9982-c7e89f2d1e75",
+    "name": "Terry Pratchett"
+  }
+]
+```
+
+Since the query simulates what the result will be after publishing the drafts, the `_id` doesn't contain the `drafts.` prefix. If you want to check if a document is a draft or not you can use the `_originalId` field, which is only available when using the `previewDrafts` perspective.
+
+```ts
+const authors = await client.fetch(`*[_type == "author"]{..., "status": select(
+  _originalId in path("drafts.**") => "draft",
+  "published"
+)}`)
+```
+
+Which changes the result to be:
+
+```json
+[
+  {
+    "_type": "author",
+    "_id": "ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "_originalId": "drafts.ecfef291-60f0-4609-bbfc-263d11a48c43",
+    "name": "George R.R. Martin",
+    "status": "draft"
+  },
+  {
+    "_type": "author",
+    "_id": "f4898efe-92c4-4dc0-9c8c-f7480aef17e2",
+    "_originalId": "f4898efe-92c4-4dc0-9c8c-f7480aef17e2",
+    "name": "Stephen King",
+    "status": "published"
+  }
+]
+```
+
 ### Fetching Content Source Maps
 
 Content Source Maps annotate fragments in your query results with metadata about its origin: the field, document, and dataset it originated from.