Add Find All Objects With A Matching Key Value Pair as a jq TIL

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

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+# 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)