Add Count Each Collection In A JSON Object as a jq TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1287 TILs and counting..._
+_1288 TILs and counting..._
 
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@@ -485,6 +485,7 @@ _1287 TILs and counting..._
 
 ### jq
 
+- [Count Each Collection In A JSON Object](jq/count-each-collection-in-a-json-object.md)
 - [Count The Number Of Things In A JSON File](jq/count-the-number-of-things-in-a-json-file.md)
 - [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)

jq/count-each-collection-in-a-json-object.md

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+# Count Each Collection In A JSON Object
+
+Let's say your JSON file is an object that represents several different
+collections (arrays) of data.
+
+```json
+{
+  "users": [ ... ],
+  "orders": [ ... ],
+  "carts": [ ... ]
+}
+```
+
+We can get a nice summary of the counts of those collections using
+[`jq`](https://stedolan.github.io/jq/). We can do that with the [`with_entries`
+function](https://stedolan.github.io/jq/manual/#to_entries,from_entries,with_entries).
+We preserve the key (name) of each collection and then process each list with
+the `length` function.
+
+```bash
+jq '. | with_entries({ "key": .key, "value": (.value | length)})' data.json
+{
+  "users": 1234,
+  "orders": 5432,
+  "carts": 89
+}
+```
+
+The `with_entries` function essentially maps over each key-value pair
+processing it with the given expression. It will then convert that `{"key":
+some_key, "value": 123}` mapping back into a key-value pair that gets combined
+with all the others.
+
+[source](https://til.simonwillison.net/jq/flatten-nested-json-objects-jq)