MONGOID-4680 implement cache_version for better Rails caching support (#5874)

* MONGOID-4680 implement cache_version for better Rails caching support

* $count was introduced after 5.0; use $sum for better compatibility

Author: jamis

lib/mongoid/association/many.rb

@@ -178,6 +178,19 @@ def unscoped
         criteria.unscoped
       end
 
+      # For compatibility with Rails' caching. Returns a string based on the
+      # given timestamp, and includes the number of records in the relation
+      # in the version.
+      #
+      # @param [ String | Symbol ] timestamp_column the timestamp column to
+      #   use when constructing the key.
+      #
+      # @return [ String ] the cache version string
+      def cache_version(timestamp_column = :updated_at)
+        @cache_version ||= {}
+        @cache_version[timestamp_column] ||= compute_cache_version(timestamp_column)
+      end
+
       private
 
       def _session
@@ -198,6 +211,54 @@ def find_or(method, attrs = {}, type = nil, &block)
         attrs[klass.discriminator_key] = type.discriminator_value if type
         where(attrs).first || send(method, attrs, type, &block)
       end
+
+      # Computes the cache version for the relation using the given
+      # timestamp colum; see `#cache_version`.
+      #
+      # @param [ String | Symbol ] timestamp_column the timestamp column to
+      #   use when constructing the key.
+      #
+      # @return [ String ] the cache version string
+      def compute_cache_version(timestamp_column)
+        timestamp_column = timestamp_column.to_s
+
+        loaded = _target.respond_to?(:_loaded?) ?
+                    _target._loaded? :   # has_many
+                    true                 # embeds_many
+
+        size, timestamp = loaded ?
+          analyze_loaded_target(timestamp_column) :
+          analyze_unloaded_target(timestamp_column)
+
+        if timestamp
+          "#{size}-#{timestamp.utc.to_formatted_s(klass.cache_timestamp_format)}"
+        else
+          size.to_s
+        end
+      end
+
+      # Return a 2-tuple of the number of elements in the relation, and the
+      # largest timestamp value.
+      def analyze_loaded_target(timestamp_column)
+        newest = _target.select { |elem| elem.respond_to?(timestamp_column) }
+                        .max { |a, b| a[timestamp_column] <=> b[timestamp_column] }
+        [ _target.length, newest ? newest[timestamp_column] : nil ]
+      end
+
+      # Returns a 2-tuple of the number of elements in the relation, and the
+      # largest timestamp value. This will query the database to perform a
+      # $sum and a $max.
+      def analyze_unloaded_target(timestamp_column)
+        pipeline = criteria
+          .group(_id: nil,
+                 count: { '$sum' => 1 },
+                 latest: { '$max' => "$#{timestamp_column}" })
+          .pipeline
+
+        result = klass.collection.aggregate(pipeline).to_a.first
+
+        result ? [ result["count"], result["latest"] ] : [ 0 ]
+      end
     end
   end
 end

lib/mongoid/cacheable.rb

@@ -15,20 +15,39 @@ module Cacheable
     # Print out the cache key. This will append different values on the
     # plural model name.
     #
-    # If new_record?     - will append /new
-    # If not             - will append /id-updated_at.to_formatted_s(cache_timestamp_format)
-    # Without updated_at - will append /id
+    # If new_record? - will append /new
+    # Non-nil cache_version? - append /id
+    # Non-nil updated_at - append /id-updated_at.to_formatted_s(cache_timestamp_format)
+    # Otherwise - append /id
     #
     # This is usually called inside a cache() block
     #
     # @example Returns the cache key
     #   document.cache_key
     #
-    # @return [ String ] the string with or without updated_at
+    # @return [ String ] the generated cache key
     def cache_key
       return "#{model_key}/new" if new_record?
+      return "#{model_key}/#{_id}" if cache_version
       return "#{model_key}/#{_id}-#{updated_at.utc.to_formatted_s(cache_timestamp_format)}" if try(:updated_at)
       "#{model_key}/#{_id}"
     end
+
+    # Return the cache version for this model. By default, it returns the updated_at
+    # field (if present) formatted as a string, or nil if the model has no
+    # updated_at field. Models with different needs may override this method to
+    # suit their desired behavior.
+    #
+    # @return [ String | nil ] the cache version value
+    #
+    # TODO: we can test this by using a MemoryStore, putting something in
+    # it, then updating the timestamp on the record and trying to read the
+    # value from the memory store. It shouldn't find it, because the version
+    # has changed.
+    def cache_version
+      if has_attribute?('updated_at') && updated_at.present?
+        updated_at.utc.to_formatted_s(cache_timestamp_format)
+      end
+    end
   end
 end

spec/integration/caching_spec.rb

@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'caching integration tests' do
+  let(:store) { ActiveSupport::Cache::MemoryStore.new }
+
+  context 'without updated_at' do
+    let(:model1) { Person.create }
+    let(:model2) { Person.create }
+
+    before do
+      store.write(model1, 'model1')
+      store.write(model2, 'model2')
+    end
+
+    it 'uses a unique key' do
+      expect(store.read(model1)).to be == 'model1'
+      expect(store.read(model2)).to be == 'model2'
+    end
+
+    context 'when updating' do
+      before do
+        model1.update title: 'updated'
+        model2.update title: 'updated'
+      end
+
+      let(:reloaded_model1) { Person.find(model1.id) }
+      let(:reloaded_model2) { Person.find(model2.id) }
+
+      it 'still finds the models' do
+        expect(store.read(reloaded_model1)).to be == 'model1'
+        expect(store.read(reloaded_model2)).to be == 'model2'
+      end
+    end
+  end
+
+  context 'with updated_at' do
+    let(:model1) { Dokument.create }
+    let(:model2) { Dokument.create }
+
+    before do
+      store.write(model1, 'model1')
+      store.write(model2, 'model2')
+    end
+
+    it 'uses a unique key' do
+      expect(store.read(model1)).to be == 'model1'
+      expect(store.read(model2)).to be == 'model2'
+    end
+
+    context 'when updating' do
+      before do
+        model1.update title: 'updated'
+        model2.update title: 'updated'
+      end
+
+      let(:reloaded_model1) { Dokument.find(model1.id) }
+      let(:reloaded_model2) { Dokument.find(model2.id) }
+
+      it 'does not find the models' do
+        # because the update caused the cache_version to change
+        expect(store.read(reloaded_model1)).to be_nil
+        expect(store.read(reloaded_model2)).to be_nil
+      end
+    end
+  end
+end

spec/mongoid/association/embedded/embeds_many/proxy_spec.rb

@@ -4896,4 +4896,105 @@ class DNS::Record
       expect(user.orders.map(&:sku).sort).to eq([ 1, 2 ])
     end
   end
+
+  describe '#cache_version' do
+    context 'when the model does not have an updated_at column' do
+      let(:root_model) { Quiz.create! }
+      let(:root) { Quiz.find(root_model.id) }
+      let(:pages) { root.pages }
+
+      let(:prepopulated_root) do
+        root_model.pages << Page.new(content: 'Page #1')
+        root_model.pages << Page.new(content: 'Page #2')
+        Quiz.find(root_model.id)
+      end
+
+      shared_examples_for 'a cache_version generator' do
+        it 'produces a trivial cache_version' do
+          expect(pages.cache_version).to be == "#{pages.length}"
+        end
+      end
+
+      context 'when the relation is empty' do
+        it_behaves_like 'a cache_version generator'
+      end
+
+      context 'when the relation is not empty' do
+        let(:root) { prepopulated_root }
+
+        it_behaves_like 'a cache_version generator'
+      end
+    end
+
+    context 'when the model has an updated_at column' do
+      let(:root_model) { Book.create(title: 'Root') }
+      let(:root) { Book.find(root_model.id) }
+
+      let(:cover) { root_model.covers.first }
+      let(:covers) { root.covers }
+      let(:original_cache_version) { root.covers.cache_version }
+
+      let(:prepopulated_root) do
+        root_model.covers << Cover.new(title: 'Cover #1')
+        root_model.covers << Cover.new(title: 'Cover #2')
+        Book.find(root_model.id)
+      end
+
+      shared_examples_for 'a cache_version generator' do
+        it 'produces a consistent cache_version' do
+          expect(covers.cache_version).not_to be_nil
+          expect(covers.cache_version).to be == covers.cache_version
+        end
+      end
+
+      context 'when the relation is empty' do
+        it_behaves_like 'a cache_version generator'
+      end
+
+      context 'when the relation is not empty' do
+        let(:root) { prepopulated_root }
+        it_behaves_like 'a cache_version generator'
+      end
+
+      context 'when an element is updated' do
+        let(:updated_cache_version) do
+          cover.update title: 'modified'
+          cover.book.save!
+          cover.book.reload.covers.cache_version
+        end
+
+        let(:root) { prepopulated_root }
+
+        it 'changes the cache_version' do
+          expect(original_cache_version).not_to be == updated_cache_version
+        end
+      end
+
+      context 'when an element is added' do
+        let(:updated_cache_version) do
+          root.covers << Cover.new(title: 'Another Cover')
+          root.reload.covers.cache_version
+        end
+
+        let(:root) { prepopulated_root }
+
+        it 'changes the cache_version' do
+          expect(original_cache_version).not_to be == updated_cache_version
+        end
+      end
+
+      context 'when an element is removed' do
+        let(:updated_cache_version) do
+          cover.destroy
+          root.reload.covers.cache_version
+        end
+
+        let(:root) { prepopulated_root }
+
+        it 'changes the cache_version' do
+          expect(original_cache_version).not_to be == updated_cache_version
+        end
+      end
+    end
+  end
 end