2 files changed, 6 insertions, 4 deletions
diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 0001b804a8..1a424896fe 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -59,7 +59,9 @@ module ActiveRecord
           r.send(reflection.association_primary_key)
         end.values_at(*ids).compact
         if records.size != ids.size
-          klass.all.raise_record_not_found_exception!(ids, records.size, ids.size, reflection.association_primary_key)
+          found_ids = records.map { |record| record.send(reflection.association_primary_key) }
+          not_found_ids = ids - found_ids
+          klass.all.raise_record_not_found_exception!(ids, records.size, ids.size, reflection.association_primary_key, not_found_ids)
         else
           replace(records)
         end
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 626c50470e..5da9573052 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -330,7 +330,7 @@ module ActiveRecord
     # of results obtained should be provided in the +result_size+ argument and
     # the expected number of results should be provided in the +expected_size+
     # argument.
-    def raise_record_not_found_exception!(ids = nil, result_size = nil, expected_size = nil, key = primary_key) # :nodoc:
+    def raise_record_not_found_exception!(ids = nil, result_size = nil, expected_size = nil, key = primary_key, not_found_ids = nil) # :nodoc:
       conditions = arel.where_sql(@klass)
       conditions = " [#{conditions}]" if conditions
       name = @klass.name
@@ -344,8 +344,8 @@ module ActiveRecord
         raise RecordNotFound.new(error, name, key, ids)
       else
         error = "Couldn't find all #{name.pluralize} with '#{key}': ".dup
-        error << "(#{ids.join(", ")})#{conditions} (found #{result_size} results, but was looking for #{expected_size})"
-
+        error << "(#{ids.join(", ")})#{conditions} (found #{result_size} results, but was looking for #{expected_size})."
+        error << " Couldn't find #{name.pluralize(not_found_ids.size)} with #{key.to_s.pluralize(not_found_ids.size)} #{not_found_ids.join(', ')}." if not_found_ids
         raise RecordNotFound.new(error, name, primary_key, ids)
       end
     end