Add support for preloading related data for missing collection entries

.ruby-version

@@ -1 +1 @@
-2.7.2
+3.1.0

README.md

@@ -76,12 +76,17 @@ end
 
     do_caching true
 
+    preloads do |cereals, options|
+      ActiveRecord::Associations::Preloader.new.preload(cereals, :maker)
+    end
+
     cache_key_addons { |options| options[:be_trendy] }
     dependency_key 'V3'
 
     serialize :uid, from: :id, to: String
     serialize :name, :company
     serialize :copyright, through: :legal_info
+    serialize :maker_name, through: :maker
     serialize :spiel do |cereal, _options|
       'Made with whole grains!' if cereal.ingredients[:whole_grains] > 0.000001
     end
@@ -136,6 +141,7 @@ CerealSerializer.new(Cereal.first, be_trendy: true, include: :ingredients).as_js
   "name": "Eyeholes",
   "company": "Needful Things",
   "copyright": "© Need Things 2019",
+  "maker_name": "Cereal Inc",
   "spiel": "Made with whole grains!",
   "tagline": "Part of a balanced breakfast",
   "small_print": "This doesn't mean jack-squat",
@@ -257,6 +263,15 @@ def fine_print
 end
 ```
 
+### Preload data to avoid n+1 issues when rendering a collection of items
+```ruby
+preloads do |cereals, options|
+  ActiveRecord::Associations::Preloader.new.preload(cereals, :maker)
+end
+```
+
+When a serializer is cached, the `cereals` will contain only those entries for which no cache was found.
+
 ### Include other data
 ```ruby
 class CerealSerializer < CacheCrispies::Base

lib/cache_crispies/base.rb

@@ -178,6 +178,15 @@ def self.file_hashes
       ).uniq.sort
     end
 
+    def self.preloads(collection = [], options = {}, &block)
+      if block_given?
+        @preloads = block
+        nil
+      else
+        @preloads&.call(collection, options)
+      end
+    end
+
     private
 
     def self.file_hash

lib/cache_crispies/collection.rb

@@ -34,6 +34,7 @@ def as_json
     attr_reader :collection, :serializer, :options
 
     def uncached_json
+      @serializer.preloads(collection, options)
       collection.map do |model|
         serializer.new(model, options).as_json
       end
@@ -46,11 +47,18 @@ def cached_json
         hash[plan.cache_key] = model
       end
 
-      CacheCrispies.cache.fetch_multi(*models_by_cache_key.keys) do |cache_key|
-        model = models_by_cache_key[cache_key]
+      already_cached = CacheCrispies.cache.read_multi(*models_by_cache_key.keys)
 
-        serializer.new(model, options).as_json
-      end.values
+      missing_keys = models_by_cache_key.keys - already_cached.keys
+      missing_values = models_by_cache_key.fetch_values(*missing_keys)
+      @serializer.preloads(missing_values, options)
+
+      new_entries = missing_keys.each_with_object({}) do |key, hash|
+        hash[key] = serializer.new(models_by_cache_key[key], options).as_json
+      end
+
+      CacheCrispies.cache.write_multi(new_entries)
+      new_entries.values + already_cached.values
     end
   end
 end

spec/cache_crispies/collection_spec.rb

@@ -58,26 +58,55 @@ def models.cache_key() end
 
         it 'fetches the cache for each object in the collection' do
           expect(CacheCrispies::Plan).to receive(:new).with(
-            serializer, model1, options
+            serializer, model1, **options
           ).and_return double('plan-dbl-1', cache_key: 'cereal-key-1')
 
           expect(CacheCrispies::Plan).to receive(:new).with(
-            serializer, model2, options
+            serializer, model2, **options
           ).and_return double('plan-dbl-2', cache_key: 'cereal-key-2')
 
+          expect(CacheCrispies).to receive_message_chain(:cache, :read_multi).
+            with('cereal-key-1', 'cereal-key-2').and_return({})
+
           expect(CacheCrispies).to receive_message_chain(
-            :cache, :fetch_multi
+            :cache, :write_multi
           ).with(
-            'cereal-key-1', 'cereal-key-2'
-          ).and_yield('cereal-key-1').and_return(
-            name: name1
-          ).and_yield('cereal-key-2').and_return(
-            name: name2
+            { 'cereal-key-1' => { name: name1 }, 'cereal-key-2' => { name: name2 } }
           )
 
           subject.as_json
         end
       end
+
+      context 'when the collection cache key does not miss' do
+        before do
+          allow(CacheCrispies).to receive_message_chain(
+            :cache, :fetch
+          ).with('cacheable-collection-key').and_yield
+        end
+
+        it 'fetches the cache for each object in the collection' do
+          expect(CacheCrispies::Plan).to receive(:new).with(
+            serializer, model1, **options
+          ).and_return double('plan-dbl-1', cache_key: 'cereal-key-1')
+
+          expect(CacheCrispies::Plan).to receive(:new).with(
+            serializer, model2, **options
+          ).and_return double('plan-dbl-2', cache_key: 'cereal-key-2')
+
+          expect(CacheCrispies).to receive_message_chain(:cache, :read_multi).
+            with('cereal-key-1', 'cereal-key-2').
+            and_return(
+              { 'cereal-key-1' => { name: name1 }, 'cereal-key-2' => { name: name2 } }
+            )
+
+          expect(CacheCrispies).to receive_message_chain(
+            :cache, :write_multi
+          ).with({})
+
+          subject.as_json
+        end
+      end
     end
   end
 end