Add Grab Values From An Array Of References as a GROQ til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1191 TILs and counting..._
+_1192 TILs and counting..._
 
 ---
 
@@ -28,6 +28,7 @@ _1191 TILs and counting..._
 * [Git](#git)
 * [GitHub Actions](#github-actions)
 * [Go](#go)
+* [GROQ](#groq)
 * [Heroku](#heroku)
 * [HTML](#html)
 * [HTTP](#http)
@@ -336,6 +337,10 @@ _1191 TILs and counting..._
 - [Sleep For A Duration](go/sleep-for-a-duration.md)
 - [Upgrading From An Older Version On Mac](go/upgrading-from-an-older-version-on-mac.md)
 
+### GROQ
+
+- [Grab Values From An Array Of References](groq/grab-values-from-an-array-of-references.md)
+
 ### Heroku
 
 - [Deploy A Review App To A Different Stack](heroku/deploy-a-review-app-to-a-different-stack.md)

groq/grab-values-from-an-array-of-references.md

@@ -0,0 +1,29 @@
+# Grab Values From An Array Of References
+
+Let's say we have a `post` object in our schema. A `post` can have an array of
+references to `tags` telling you what topics the post covers. Each `tag` has a
+slug and we want to get the `string` value for each slug.
+
+Let's say we are interested in the post with `_id` of `123`.
+
+Here is how we can achieve that with a `groq` query:
+
+```groq
+*[
+  _type == 'post' && _id == 123
+]{
+  'tags': tags[]->slug.current
+}.tags
+
+=> ["javascript", "react-js"]
+```
+
+If the schema was such that each `post` just had a single tag reference, then
+you could write the chain of references as `tag->slug.current`. Because it is
+an array of references, we need the `[]` to declare that we want each value.
+
+The `->` operator follows the reference. Otherwise we'd just have access to the
+`_ref` and `_type` values.
+
+The final `.tags` unnests the `tags` value we gathered into an object. Then the
+result is just the array of slug values.