Add comment that explains what happens

Author: dennispaagman

app/models/solid_cache/entry.rb

@@ -92,6 +92,17 @@ def upsert_unique_by
           connection.supports_insert_conflict_target? ? :key_hash : nil
         end
 
+        # This constructs and caches a SQL query for a given number of keys.
+        #
+        # The query is constructed with two bind parameters to generate an IN (...) condition,
+        # which is then replaced with the correct amount based on the number of keys. The
+        # parameters are filled later when executing the query. This is done through Active Record
+        # to ensure the field and table names are properly quoted and escaped based on the used database adapter.
+
+        # For example: The query for 4 keys will be transformed from:
+        # > SELECT "key", "value" FROM "solid_cache_entries" WHERE "key_hash" IN (1111, 2222)
+        # into:
+        # > SELECT "key", "value" FROM "solid_cache_entries" WHERE "key_hash" IN (?, ?, ?, ?)
         def select_sql(keys)
           @select_sql ||= {}
           @select_sql[keys.count] ||= \