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]]

3 files changed, 4 insertions, 4 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