Expand section for advanced serialization

README.md

@@ -864,7 +864,75 @@ The implementation of `inline_serializer` can be found [here](https://github.com
 
 ### Advanced serialization
 
-*Coming soon*
+Sometimes, the end result of an API can be quite complex. Sometimes, we want to optimize the queries that we do and the optimization itself is quite complex.
+
+Trying to stick with just an `OutputSerializer` in that case might limit our options.
+
+In those cases, we can implement our output serialization as a function, and have the optimizations we need there.
+
+Lets take this API as an example:
+
+```python
+class SomeGenericFeedApi(BaseApi):
+    def get(self, request):
+        feed = some_feed_get(
+            user=request.user,
+        )
+
+        data = some_feed_serialize(feed)
+
+        return Response(data)
+```
+
+In this scenario, `some_feed_get` has the responsibility of returning a list of feed items (can be ORM objects, can be just IDs).
+
+And we want to push the complexity of serializing this feed, in an optimal manner, to the serializer function - `some_feed_serialize`.
+
+This means we don't have to do any unnecessary prefetches & optimizations in `some_feed_get`.
+
+Here's an example of `some_feed_serialize`:
+
+```python
+class FeedItemSerializer(serializers.Serializer):
+    ... some fields here ...
+    calculated_field = serializers.IntegerField(source="_calculated_field")
+
+
+def some_feed_serialize(feed: List[FeedItem]):
+    feed_ids = [feed_item.id for feed_item in feed]
+
+    # Refetch items with more optimizations
+    # Based on the relations that are going in
+    objects = FeedItem.objects.select_related(
+      ... as complex as you want ...
+    ).prefetch_related(
+      ... as complex as you want ...
+    ).filter(
+      id__in=feed_ids
+    ).order_by(
+      "-some_timestamp"
+    )
+
+    some_cache = get_some_cache(feed_ids)
+
+    result = []
+
+    for feed_item in objects:
+        # An example, adding additional fields for the serializer
+        # That are based on values outside of our current object
+        # This may be some optimization to save queries
+        feed_item._calculated_field = some_cache.get(feed_item.id)
+
+        result.append(FeedItemSerializer(feed_item).data)
+
+    return result
+```
+
+As you can see, this is a pretty generic example, but the idea is simple:
+
+1. Refetch your data, with the needed joins & prefetches.
+1. Fetch or build in-memory caches, that will save you queries for specific computed values.
+1. Return a result, that's ready to be an API response.
 
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