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]]

6 files changed, 9 insertions, 9 deletions

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)