5 files changed, 68 insertions, 1 deletions
diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index f5cf92db64..264f869c68 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -53,6 +53,7 @@ module ActiveRecord
   autoload :Persistence
   autoload :QueryCache
   autoload :Querying
+  autoload :CollectionCacheKey
   autoload :ReadonlyAttributes
   autoload :RecordInvalid, 'active_record/validations'
   autoload :Reflection
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index abe1d465a5..7fb899c242 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -230,7 +230,15 @@ module ActiveRecord
     #   person.respond_to(:nothing) # => false
     def respond_to?(name, include_private = false)
       return false unless super
-      name = name.to_s
+
+      case name
+      when :to_partial_path
+        name = "to_partial_path".freeze
+      when :to_model
+        name = "to_model".freeze
+      else
+        name = name.to_s
+      end
 
       # If the result is true then check for the select case.
       # For queries selecting a subset of columns, return false for unselected columns.
diff --git a/activerecord/lib/active_record/base.rb b/activerecord/lib/active_record/base.rb
index c918e88590..55a7e053bc 100644
--- a/activerecord/lib/active_record/base.rb
+++ b/activerecord/lib/active_record/base.rb
@@ -280,6 +280,7 @@ module ActiveRecord #:nodoc:
     extend Explain
     extend Enum
     extend Delegation::DelegateCache
+    extend CollectionCacheKey
 
     include Core
     include Persistence
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..3c4ca3d116
--- /dev/null
+++ b/activerecord/lib/active_record/collection_cache_key.rb
@@ -0,0 +1,31 @@
+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")
+          .unscope(:order)
+        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
diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index e4df122af6..3ed04dee3b 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -298,6 +298,32 @@ module ActiveRecord
       limit_value ? to_a.many? : size > 1
     end
 
+    # Returns a cache key that can be used to identify the records fetched by
+    # this query. The cache key is built with a fingerprint of the sql query,
+    # the number of records matched by the query and a timestamp of the last
+    # updated record. When a new record comes to match the query, or any of
+    # the existing records is updated or deleted, the cache key changes.
+    #
+    #   Product.where("name like ?", "%Cosmic Encounter%").cache_key
+    #   => "products/query-1850ab3d302391b85b8693e941286659-1-20150714212553907087000"
+    #
+    # If the collection is loaded, the method will iterate through the records
+    # to generate the timestamp, otherwise it will trigger one SQL query like:
+    #
+    #    SELECT COUNT(*), MAX("products"."updated_at") FROM "products" WHERE (name like '%Cosmic Encounter%')
+    #
+    # You can also pass a custom timestamp column to fetch the timestamp of the
+    # last updated record.
+    #
+    #   Product.where("name like ?", "%Game%").cache_key(:last_reviewed_at)
+    #
+    # You can customize the strategy to generate the key on a per model basis
+    # overriding ActiveRecord::Base#collection_cache_key.
+    def cache_key(timestamp_column = :updated_at)
+      @cache_keys ||= {}
+      @cache_keys[timestamp_column] ||= @klass.collection_cache_key(self, timestamp_column)
+    end
+
     # Scope all queries to the current scope.
     #
     #   Comment.where(post_id: 1).scoping do