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]]

1 files changed, 2 insertions, 2 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)