Revert "Use flat_map { } instead of map {}.flatten"

This reverts commit abf8de85519141496a6773310964ec03f6106f3f.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.

irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]

17 files changed, 26 insertions, 26 deletions

diff --git a/actionpack/lib/action_dispatch/routing/inspector.rb b/actionpack/lib/action_dispatch/routing/inspector.rb
index 178e2074bc..c18dc94d4f 100644
--- a/actionpack/lib/action_dispatch/routing/inspector.rb
+++ b/actionpack/lib/action_dispatch/routing/inspector.rb
@@ -102,9 +102,9 @@ module ActionDispatch
       end
 
       def formatted_routes_for_engines
-        @engines.flat_map do |name, routes|
+        @engines.map do |name, routes|
           ["\nRoutes for #{name}:"] + formatted_routes(routes)
-        end
+        end.flatten
       end
 
       def formatted_routes(routes)
diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index 16491cbee7..cd366ac8b7 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -35,12 +35,12 @@ module ActiveRecord
       end
 
       def columns
-        join_parts.flat_map { |join_part|
+        join_parts.collect { |join_part|
           table = join_part.aliased_table
           join_part.column_names_with_alias.collect{ |column_name, aliased_name|
             table[column_name].as Arel.sql(aliased_name)
           }
-        }
+        }.flatten
       end
 
       def instantiate(rows)
diff --git a/activerecord/lib/active_record/associations/preloader.rb b/activerecord/lib/active_record/associations/preloader.rb
index 9cbc117945..ce5bf15f10 100644
--- a/activerecord/lib/active_record/associations/preloader.rb
+++ b/activerecord/lib/active_record/associations/preloader.rb
@@ -106,7 +106,7 @@ module ActiveRecord
       def preload_hash(association)
         association.each do |parent, child|
           Preloader.new(records, parent, preload_scope).run
-          Preloader.new(records.flat_map { |record| record.send(parent) }, child).run
+          Preloader.new(records.map { |record| record.send(parent) }.flatten, child).run
         end
       end
 
diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index 358d84513c..cbf5e734ea 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -77,7 +77,7 @@ module ActiveRecord
             # Some databases impose a limit on the number of ids in a list (in Oracle it's 1000)
             # Make several smaller queries if necessary or make one query if the adapter supports it
             sliced  = owner_keys.each_slice(model.connection.in_clause_length || owner_keys.size)
-            records = sliced.flat_map { |slice| records_for(slice).to_a }
+            records = sliced.map { |slice| records_for(slice).to_a }.flatten
           end
 
           # Each record may have multiple owners, and vice-versa
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index dcb89178f7..8c83c4f5db 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -448,7 +448,7 @@ module ActiveRecord
       end
 
       def bulk_change_table(table_name, operations) #:nodoc:
-        sqls = operations.flat_map do |command, args|
+        sqls = operations.map do |command, args|
           table, arguments = args.shift, args
           method = :"#{command}_sql"
 
@@ -457,7 +457,7 @@ module ActiveRecord
           else
             raise "Unknown method called : #{method}(#{arguments.inspect})"
           end
-        end.join(", ")
+        end.flatten.join(", ")
 
         execute("ALTER TABLE #{quote_table_name(table_name)} #{sqls}")
       end
diff --git a/activerecord/lib/active_record/observer.rb b/activerecord/lib/active_record/observer.rb
index 6fe98a8f6d..6b2f6f98a5 100644
--- a/activerecord/lib/active_record/observer.rb
+++ b/activerecord/lib/active_record/observer.rb
@@ -99,7 +99,7 @@ module ActiveRecord
 
       def observed_classes
         klasses = super
-        klasses + klasses.flat_map { |klass| klass.descendants }
+        klasses + klasses.map { |klass| klass.descendants }.flatten
       end
 
       def add_observer!(klass)
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 21c2b02b7e..3c59bd8a68 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -782,7 +782,7 @@ module ActiveRecord
     def reverse_sql_order(order_query)
       order_query = ["#{quoted_table_name}.#{quoted_primary_key} ASC"] if order_query.empty?
 
-      order_query.flat_map do |o|
+      order_query.map do |o|
         case o
         when Arel::Nodes::Ordering
           o.reverse
@@ -794,7 +794,7 @@ module ActiveRecord
         else
           o
         end
-      end
+      end.flatten
     end
 
     def array_of_strings?(o)
diff --git a/activerecord/test/cases/associations/callbacks_test.rb b/activerecord/test/cases/associations/callbacks_test.rb
index 0463b7d046..2d0d4541b4 100644
--- a/activerecord/test/cases/associations/callbacks_test.rb
+++ b/activerecord/test/cases/associations/callbacks_test.rb
@@ -138,7 +138,7 @@ class AssociationCallbacksTest < ActiveRecord::TestCase
       activerecord.reload
       assert activerecord.developers_with_callbacks.size == 2
     end
-    log_array = activerecord.developers_with_callbacks.flat_map {|d| ["before_removing#{d.id}","after_removing#{d.id}"]}.sort
+    log_array = activerecord.developers_with_callbacks.collect {|d| ["before_removing#{d.id}","after_removing#{d.id}"]}.flatten.sort
     assert activerecord.developers_with_callbacks.clear
     assert_equal log_array, activerecord.developers_log.sort
   end
diff --git a/activerecord/test/cases/associations/has_many_through_associations_test.rb b/activerecord/test/cases/associations/has_many_through_associations_test.rb
index c61ed61f1e..f0582a3090 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -597,7 +597,7 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     sarah = Person.create!(:first_name => 'Sarah', :primary_contact_id => people(:susan).id, :gender => 'F', :number1_fan_id => 1)
     john = Person.create!(:first_name => 'John', :primary_contact_id => sarah.id, :gender => 'M', :number1_fan_id => 1)
     assert_equal sarah.agents, [john]
-    assert_equal people(:susan).agents.flat_map(&:agents), people(:susan).agents_of_agents
+    assert_equal people(:susan).agents.map(&:agents).flatten, people(:susan).agents_of_agents
   end
 
   def test_associate_existing_with_nonstandard_primary_key_on_belongs_to
diff --git a/activerecord/test/cases/autosave_association_test.rb b/activerecord/test/cases/autosave_association_test.rb
index 8d10fe5fdf..fd4f09ab36 100644
--- a/activerecord/test/cases/autosave_association_test.rb
+++ b/activerecord/test/cases/autosave_association_test.rb
@@ -50,9 +50,9 @@ class TestAutosaveAssociationsInGeneral < ActiveRecord::TestCase
   end
 
   def callbacks_for_model(model)
-    model.instance_variables.grep(/_callbacks$/).flat_map do |ivar|
+    model.instance_variables.grep(/_callbacks$/).map do |ivar|
       model.instance_variable_get(ivar)
-    end
+    end.flatten
   end
 end
 
diff --git a/activesupport/lib/active_support/multibyte/unicode.rb b/activesupport/lib/active_support/multibyte/unicode.rb
index d7a403c45b..f49ca47f14 100644
--- a/activesupport/lib/active_support/multibyte/unicode.rb
+++ b/activesupport/lib/active_support/multibyte/unicode.rb
@@ -219,9 +219,9 @@ module ActiveSupport
       # encoding is entirely CP1252 or ISO-8859-1.
       def tidy_bytes(string, force = false)
         if force
-          return string.unpack("C*").flat_map do |b|
+          return string.unpack("C*").map do |b|
             tidy_byte(b)
-          end.compact.pack("C*").unpack("U*").pack("U*")
+          end.flatten.compact.pack("C*").unpack("U*").pack("U*")
         end
 
         bytes = string.unpack("C*")
diff --git a/guides/rails_guides/helpers.rb b/guides/rails_guides/helpers.rb
index c8ab9e4536..e6ef656474 100644
--- a/guides/rails_guides/helpers.rb
+++ b/guides/rails_guides/helpers.rb
@@ -17,7 +17,7 @@ module RailsGuides
     end
 
     def documents_flat
-      documents_by_section.flat_map {|section| section['documents']}
+      documents_by_section.map {|section| section['documents']}.flatten
     end
 
     def finished_documents(documents)
diff --git a/guides/source/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index 1930cbe22c..2a84242b9c 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -517,7 +517,7 @@ ActionController::TestCase.class_eval do
 
   # now redefine process and delegate to original_process
   def process(action, params=nil, session=nil, flash=nil, http_method='GET')
-    params = Hash[*params.flat_map {|k, v| [k, v.to_s]}]
+    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
     original_process(action, params, session, flash, http_method)
   end
 end
@@ -530,7 +530,7 @@ That technique has a risk, it could be the case that `:original_process` was tak
 ```ruby
 ActionController::TestCase.class_eval do
   def process_with_stringified_params(...)
-    params = Hash[*params.flat_map {|k, v| [k, v.to_s]}]
+    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
     process_without_stringified_params(action, params, session, flash, http_method)
   end
   alias_method :process_without_stringified_params, :process
@@ -543,7 +543,7 @@ The method `alias_method_chain` provides a shortcut for that pattern:
 ```ruby
 ActionController::TestCase.class_eval do
   def process_with_stringified_params(...)
-    params = Hash[*params.flat_map {|k, v| [k, v.to_s]}]
+    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
     process_without_stringified_params(action, params, session, flash, http_method)
   end
   alias_method_chain :process, :stringified_params
diff --git a/railties/lib/rails/generators/actions.rb b/railties/lib/rails/generators/actions.rb
index 735a1972fe..c41acc7841 100644
--- a/railties/lib/rails/generators/actions.rb
+++ b/railties/lib/rails/generators/actions.rb
@@ -184,7 +184,7 @@ module Rails
       #   generate(:authenticated, "user session")
       def generate(what, *args)
         log :generate, what
-        argument = args.flat_map {|arg| arg.to_s }.join(" ")
+        argument = args.map {|arg| arg.to_s }.flatten.join(" ")
 
         in_root { run_ruby_script("script/rails generate #{what} #{argument}", :verbose => false) }
       end
diff --git a/railties/lib/rails/paths.rb b/railties/lib/rails/paths.rb
index 30482fba5d..3c2210aaf9 100644
--- a/railties/lib/rails/paths.rb
+++ b/railties/lib/rails/paths.rb
@@ -103,7 +103,7 @@ module Rails
         all_paths.each do |path|
           if path.send(constraint)
             paths  = path.existent
-            paths -= path.children.flat_map { |p| p.send(constraint) ? [] : p.existent }
+            paths -= path.children.map { |p| p.send(constraint) ? [] : p.existent }.flatten
             all.concat(paths)
           end
         end
diff --git a/railties/lib/rails/source_annotation_extractor.rb b/railties/lib/rails/source_annotation_extractor.rb
index 6a8b09dbb7..31e34023c0 100644
--- a/railties/lib/rails/source_annotation_extractor.rb
+++ b/railties/lib/rails/source_annotation_extractor.rb
@@ -100,7 +100,7 @@ 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.flat_map { |f, a| a.map(&:line) }.max.to_s.size
+    options[:indent] = results.map { |f, a| a.map(&:line) }.flatten.max.to_s.size
     results.keys.sort.each do |file|
       puts "#{file}:"
       results[file].each do |note|
diff --git a/railties/lib/rails/test_unit/testing.rake b/railties/lib/rails/test_unit/testing.rake
index d1d4230132..0de4afe905 100644
--- a/railties/lib/rails/test_unit/testing.rake
+++ b/railties/lib/rails/test_unit/testing.rake
@@ -6,7 +6,7 @@ TEST_CHANGES_SINCE = Time.now - 600
 
 # Look up tests for recently modified sources.
 def recent_tests(source_pattern, test_path, touched_since = 10.minutes.ago)
-  FileList[source_pattern].flat_map do |path|
+  FileList[source_pattern].map do |path|
     if File.mtime(path) > touched_since
       tests = []
       source_dir = File.dirname(path).split("/")
@@ -26,7 +26,7 @@ def recent_tests(source_pattern, test_path, touched_since = 10.minutes.ago)
       return tests
 
     end
-  end.compact
+  end.flatten.compact
 end