diff --git a/railties/lib/rails/commands/plugin.rb b/railties/lib/rails/commands/plugin.rb
index f7a0b99005..95bbdd4cdf 100644
--- a/railties/lib/rails/commands/plugin.rb
+++ b/railties/lib/rails/commands/plugin.rb
@@ -11,7 +11,7 @@ else
               end
     if File.exist?(railsrc)
       extra_args_string = File.read(railsrc)
-      extra_args = extra_args_string.split(/\n+/).map {|l| l.split}.flatten
+      extra_args = extra_args_string.split(/\n+/).flat_map {|l| l.split}
       puts "Using #{extra_args.join(" ")} from #{railsrc}"
       ARGV.insert(1, *extra_args)
     end
diff --git a/railties/lib/rails/generators/actions.rb b/railties/lib/rails/generators/actions.rb
index afdbf5c241..625f031c94 100644
--- a/railties/lib/rails/generators/actions.rb
+++ b/railties/lib/rails/generators/actions.rb
@@ -188,7 +188,7 @@ module Rails
       #   generate(:authenticated, "user session")
       def generate(what, *args)
         log :generate, what
-        argument = args.map {|arg| arg.to_s }.flatten.join(" ")
+        argument = args.flat_map {|arg| arg.to_s }.join(" ")
 
         in_root { run_ruby_script("bin/rails generate #{what} #{argument}", verbose: false) }
       end
diff --git a/railties/lib/rails/paths.rb b/railties/lib/rails/paths.rb
index 117bb37487..3eb66c07af 100644
--- a/railties/lib/rails/paths.rb
+++ b/railties/lib/rails/paths.rb
@@ -101,7 +101,7 @@ module Rails
       def filter_by(&block)
         all_paths.find_all(&block).flat_map { |path|
           paths = path.existent
-          paths - path.children.map { |p| yield(p) ? [] : p.existent }.flatten
+          paths - path.children.flat_map { |p| yield(p) ? [] : p.existent }
         }.uniq
       end
     end
diff --git a/railties/lib/rails/source_annotation_extractor.rb b/railties/lib/rails/source_annotation_extractor.rb
index 3cf6a005ea..83e28090f8 100644
--- a/railties/lib/rails/source_annotation_extractor.rb
+++ b/railties/lib/rails/source_annotation_extractor.rb
@@ -115,7 +115,7 @@ class SourceAnnotationExtractor
   # Prints the mapping from filenames to annotations in +results+ ordered by filename.
   # The +options+ hash is passed to each annotation's +to_s+.
   def display(results, options={})
-    options[:indent] = results.map { |f, a| a.map(&:line) }.flatten.max.to_s.size
+    options[:indent] = results.flat_map { |f, a| a.map(&:line) }.max.to_s.size
     results.keys.sort.each do |file|
       puts "#{file}:"
       results[file].each do |note|