Cache columns for has_and_belongs_to_many associations

This avoids repeatedly calling SHOW COLUMNS when the association is queried
[#1738 state:committed]

1 files changed, 9 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb b/activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb
index 3d689098b5..a5cc3bf091 100644
--- a/activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb
+++ b/activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb
@@ -9,6 +9,14 @@ module ActiveRecord
         create_record(attributes) { |record| insert_record(record, true) }
       end
 
+      def columns
+        @reflection.columns(@reflection.options[:join_table], "#{@reflection.options[:join_table]} Columns")
+      end
+
+      def reset_column_information
+        @reflection.reset_column_information
+      end
+
       protected
         def construct_find_options!(options)
           options[:joins]      = @join_sql
@@ -32,8 +40,6 @@ module ActiveRecord
           if @reflection.options[:insert_sql]
             @owner.connection.insert(interpolate_sql(@reflection.options[:insert_sql], record))
           else
-            columns = @owner.connection.columns(@reflection.options[:join_table], "#{@reflection.options[:join_table]} Columns")
-
             attributes = columns.inject({}) do |attrs, column|
               case column.name.to_s
                 when @reflection.primary_key_name.to_s
@@ -103,7 +109,7 @@ module ActiveRecord
         # clause has been explicitly defined. Otherwise you can get broken records back, if, for example, the join column also has
         # an id column. This will then overwrite the id column of the records coming back.
         def finding_with_ambiguous_select?(select_clause)
-          !select_clause && @owner.connection.columns(@reflection.options[:join_table], "Join Table Columns").size != 2
+          !select_clause && columns.size != 2
         end
 
       private