Cache columns for has_and_belongs_to_many associations

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

activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb

@@ -9,6 +9,14 @@ def create!(attributes = {})
         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 @@ def insert_record(record, force=true)
           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 @@ def construct_scope
         # 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

activerecord/lib/active_record/reflection.rb

@@ -198,6 +198,14 @@ def counter_cache_column
         end
       end
 
+      def columns(tbl_name, log_msg)
+        @columns ||= klass.connection.columns(tbl_name, log_msg)
+      end
+
+      def reset_column_information
+        @columns = nil
+      end
+
       def check_validity!
       end
 

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -770,4 +770,15 @@ def test_association_proxy_transaction_method_starts_transaction_in_association_
       end
     end
   end
+
+  def test_caching_of_columns
+    david = Developer.find(1)
+    # clear cache possibly created by other tests
+    david.projects.reset_column_information
+    assert_queries(1) { david.projects.columns; david.projects.columns }
+    # and again to verify that reset_column_information clears the cache correctly
+    david.projects.reset_column_information
+    assert_queries(1) { david.projects.columns; david.projects.columns }
+  end
+
 end