Add a FAQ entry about splitting items in item pipelines

docs/faq.rst

@@ -319,6 +319,29 @@ I'm scraping a XML document and my XPath selector doesn't return any items
 
 You may need to remove namespaces. See :ref:`removing-namespaces`.
 
+How to split an item into multiple items in an item pipeline?
+-------------------------------------------------------------
+
+:ref:`Item pipelines <topics-item-pipeline>` cannot yield multiple items per
+input item. :ref:`Create a spider middleware <custom-spider-middleware>`
+instead, and use its
+:meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_output`
+method for this puspose. For example::
+
+    from copy import deepcopy
+
+    from scrapy.item import BaseItem
+
+
+    class MultiplyItemsMiddleware:
+
+        def process_spider_output(self, response, result, spider):
+            for item in result:
+                if isinstance(item, (BaseItem, dict)):
+                    for _ in range(item['multiply_by']):
+                        yield deepcopy(item)
+
+
 .. _user agents: https://en.wikipedia.org/wiki/User_agent
 .. _LIFO: https://en.wikipedia.org/wiki/Stack_(abstract_data_type)
 .. _DFO order: https://en.wikipedia.org/wiki/Depth-first_search

docs/topics/spider-middleware.rst

@@ -54,6 +54,8 @@ value.  For example, if you want to disable the off-site middleware::
 Finally, keep in mind that some middlewares may need to be enabled through a
 particular setting. See each middleware documentation for more info.
 
+.. _custom-spider-middleware:
+
 Writing your own spider middleware
 ==================================