Handle no serializer source file to digest.

output warning

Closes rails-api#1176

lib/active_model/serializer.rb

@@ -29,6 +29,20 @@ class Serializer
       )
     /x
 
+    # Hashes contents of file for +_cache_digest+
+    def self.digest_caller_file(caller_line)
+      serializer_file_path = caller_line[CALLER_FILE]
+      serializer_file_contents = IO.read(serializer_file_path)
+      Digest::MD5.hexdigest(serializer_file_contents)
+    rescue TypeError, Errno::ENOENT
+      warn <<-EOF.strip_heredoc
+       Cannot digest non-existent file: '#{caller_line}'.
+       Please set `::_cache_digest` of the serializer
+       if you'd like to cache it.
+       EOF
+      ''.freeze
+    end
+
     class << self
       attr_accessor :_attributes # @api private : names of attribute methods, @see Serializer#attribute
       attr_accessor :_attributes_keys # @api private : maps attribute value to explict key name, @see Serializer#attribute
@@ -54,9 +68,10 @@ class << self
     # Serializers inherit _attributes and _attributes_keys.
     # Generates a unique digest for each serializer at load.
     def self.inherited(base)
+      caller_line = caller.first
       base._attributes = _attributes.try(:dup) || []
       base._attributes_keys = _attributes_keys.try(:dup) || {}
-      base._cache_digest = digest_caller_file(caller.first)
+      base._cache_digest = digest_caller_file(caller_line)
       super
     end
 
@@ -163,13 +178,6 @@ def self.serializers_cache
       @serializers_cache ||= ThreadSafe::Cache.new
     end
 
-    # Hashes contents of file for +_cache_digest+
-    def self.digest_caller_file(caller_line)
-      serializer_file_path = caller_line[CALLER_FILE]
-      serializer_file_contents = IO.read(serializer_file_path)
-      Digest::MD5.hexdigest(serializer_file_contents)
-    end
-
     # Find a serializer from a class and caches the lookup.
     # Preferentially retuns:
     #   1. class name appended with "Serializer"

test/serializers/cache_test.rb

@@ -3,6 +3,8 @@
 module ActiveModel
   class Serializer
     class CacheTest < Minitest::Test
+      include ActiveSupport::Testing::Stream
+
       def setup
         ActionController::Base.cache_store.clear
         @comment        = Comment.new(id: 1, body: 'ZOMG A COMMENT')
@@ -170,6 +172,16 @@ def test_digest_caller_file
         file.unlink
       end
 
+      def test_warn_on_serializer_not_defined_in_file
+        called = false
+        serializer = Class.new(ActiveModel::Serializer)
+        assert_match(/_cache_digest/, (capture(:stderr) do
+          serializer.digest_caller_file('')
+          called = true
+        end))
+        assert called
+      end
+
       private
 
       def render_object_with_cache(obj)