Use to_f for hash_cache_field to get millisecond precision for Active…

…Support::TimeWithZone timestamps

lib/jsonapi/basic_resource.rb

@@ -1019,7 +1019,14 @@ def attribute_caching_context(_context)
 
       # Generate a hashcode from the value to be used as part of the cache lookup
       def hash_cache_field(value)
-        Digest::MD5.hexdigest(value.to_s)
+        if value.nil?
+          nil
+        elsif value.is_a?(ActiveSupport::TimeWithZone)
+          # for timestamps use to_f instead of computing a hash
+          value.to_f
+        else
+          Digest::MD5.hexdigest(value)
+        end
       end
 
       def _model_class

test/fixtures/posts.yml

@@ -3,6 +3,8 @@ post_1:
   title: New post
   body: A body!!!
   author_id: 1001
+  created_at: 2020-01-01 14:15:16 UTC
+  updated_at: 2020-01-15 14:15:12 UTC
 
 post_2:
   id: 2

test/unit/resource/resource_test.rb

@@ -95,7 +95,7 @@ class PostWithReadonlyAttributesResource < JSONAPI::Resource
 
 class ResourceTest < ActiveSupport::TestCase
   def setup
-    @post = Post.first
+    @post = posts(:post_1)
   end
 
   def test_model_name
@@ -126,6 +126,33 @@ def test_resource_for_resource_does_not_exist_at_root
     end
   end
 
+  def test_cache_ids_are_consistent_from_run_to_run
+    assert_equal '2020-01-15 14:15:12 UTC', @post.updated_at.to_s
+
+    post_resource = PostResource.new(@post, {})
+    assert_equal [@post.id, 1579097712.0], post_resource.cache_id
+  end
+
+  def test_cache_ids_change_when_cache_field_is_changed
+    assert_equal '2020-01-15 14:15:12 UTC', @post.updated_at.to_s
+
+    post_resource = PostResource.new(@post, {})
+    assert_equal [@post.id, 1579097712.0], post_resource.cache_id
+
+    assert @post.touch
+    refute_equal [@post.id, 1579097712.0], post_resource.cache_id
+  end
+
+  def test_date_based_cache_ids_change_with_milliseconds_if_hash_cache_field_is_time_stamp
+    assert_equal '2020-01-15 14:15:12 UTC', @post.updated_at.to_s
+
+    post_resource = PostResource.new(@post, {})
+    assert_equal [@post.id, 1579097712.0], post_resource.cache_id
+
+    @post.updated_at = @post.updated_at + 0.222
+    refute_equal [@post.id, 1579097712.0], post_resource.cache_id
+  end
+
   def test_resource_for_with_underscored_namespaced_paths
     assert_equal(JSONAPI::Resource.resource_klass_for('my_module/related'), MyModule::RelatedResource)
     assert_equal(PostResource.resource_klass_for('my_module/related'), MyModule::RelatedResource)