Add Find All Objects With A Matching Key Value Pair as a jq 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).
 
-_1274 TILs and counting..._
+_1275 TILs and counting..._
 
 ---
 
@@ -478,6 +478,7 @@ _1274 TILs and counting..._
 ### jq
 
 - [Extract A List Of Values](jq/extract-a-list-of-values.md)
+- [Find All Objects With A Matching Key Value Pair](jq/find-all-objects-with-a-matching-key-value-pair.md)
 - [Reduce Object To Just Entries Of A Specific Type](jq/reduce-object-to-just-entries-of-a-specific-type.md)
 
 ### Kitty

jq/find-all-objects-with-a-matching-key-value-pair.md

@@ -0,0 +1,45 @@
+# Find All Objects With A Matching Key Value Pair
+
+Let's say I have a JSON file representing a bunch of people's reading lists.
+That means it is an array of objects where each object is a person's reading
+profile and contains a _list_ of books. Some of those books have a _status_ of
+`reading` meaning the person is currently reading that book.
+
+How can we find all books that are currently being read?
+
+```
+jq '. | map(
+      {
+        name: .username,
+        in_progress_books: (.books | map(select(.status == "reading")))
+      }
+    )'
+```
+
+```
+[
+  { name: 'bobr', in_progress_books: [...] },
+  { name: 'sallyf', in_progress_books: [...] },
+  ...
+]
+```
+
+That will show us for each reader what books they are currently reading.
+
+Alternatively, we could roll that all up into a single list of books.
+
+```
+jq '. |
+      map(.books | map(select(.status == "reading"))) |
+      flatten'
+```
+
+```
+[
+  { title: 'Moby Dick', status: 'reading', ... }
+  { title: 'The Great Gatsby', status: 'reading', ... }
+  ...
+]
+```
+
+[source](https://stackoverflow.com/a/18608100/535590)