Add #cache_key to ActiveRecord::Relation.

1 files changed, 29 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/collection_cache_key.rb b/activerecord/lib/active_record/collection_cache_key.rb
new file mode 100644
index 0000000000..72b50c1d28
--- /dev/null
+++ b/activerecord/lib/active_record/collection_cache_key.rb
@@ -0,0 +1,29 @@
+module ActiveRecord
+  module CollectionCacheKey
+
+    def collection_cache_key(collection = all, timestamp_column = :updated_at) # :nodoc:
+      query_signature = Digest::MD5.hexdigest(collection.to_sql)
+      key = "#{collection.model_name.cache_key}/query-#{query_signature}"
+
+      if collection.loaded?
+        size = collection.size
+        timestamp = collection.max_by(&timestamp_column).public_send(timestamp_column)
+      else
+        column_type = type_for_attribute(timestamp_column.to_s)
+        column = "#{connection.quote_table_name(collection.table_name)}.#{connection.quote_column_name(timestamp_column)}"
+
+        query = collection.select("COUNT(*) AS size", "MAX(#{column}) AS timestamp")
+        result = connection.select_one(query)
+
+        size = result["size"]
+        timestamp = column_type.deserialize(result["timestamp"])
+      end
+
+      if timestamp
+        "#{key}-#{size}-#{timestamp.utc.to_s(cache_timestamp_format)}"
+      else
+        "#{key}-#{size}"
+      end
+    end
+  end
+end