1 files changed, 78 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations/join_dependency/join_part.rb b/activerecord/lib/active_record/associations/join_dependency/join_part.rb
new file mode 100644
index 0000000000..3279e56e7d
--- /dev/null
+++ b/activerecord/lib/active_record/associations/join_dependency/join_part.rb
@@ -0,0 +1,78 @@
+module ActiveRecord
+  module Associations
+    class JoinDependency # :nodoc:
+      # A JoinPart represents a part of a JoinDependency. It is an abstract class, inherited
+      # by JoinBase and JoinAssociation. A JoinBase represents the Active Record which
+      # everything else is being joined onto. A JoinAssociation represents an association which
+      # is joining to the base. A JoinAssociation may result in more than one actual join
+      # operations (for example a has_and_belongs_to_many JoinAssociation would result in
+      # two; one for the join table and one for the target table).
+      class JoinPart # :nodoc:
+        # The Active Record class which this join part is associated 'about'; for a JoinBase
+        # this is the actual base model, for a JoinAssociation this is the target model of the
+        # association.
+        attr_reader :active_record
+
+        delegate :table_name, :column_names, :primary_key, :reflections, :arel_engine, :to => :active_record
+
+        def initialize(active_record)
+          @active_record = active_record
+          @cached_record = {}
+          @column_names_with_alias = nil
+        end
+
+        def aliased_table
+          Arel::Nodes::TableAlias.new aliased_table_name, table
+        end
+
+        def ==(other)
+          raise NotImplementedError
+        end
+
+        # An Arel::Table for the active_record
+        def table
+          raise NotImplementedError
+        end
+
+        # The prefix to be used when aliasing columns in the active_record's table
+        def aliased_prefix
+          raise NotImplementedError
+        end
+
+        # The alias for the active_record's table
+        def aliased_table_name
+          raise NotImplementedError
+        end
+
+        # The alias for the primary key of the active_record's table
+        def aliased_primary_key
+          "#{aliased_prefix}_r0"
+        end
+
+        # An array of [column_name, alias] pairs for the table
+        def column_names_with_alias
+          unless @column_names_with_alias
+            @column_names_with_alias = []
+
+            ([primary_key] + (column_names - [primary_key])).each_with_index do |column_name, i|
+              @column_names_with_alias << [column_name, "#{aliased_prefix}_r#{i}"]
+            end
+          end
+          @column_names_with_alias
+        end
+
+        def extract_record(row)
+          Hash[column_names_with_alias.map{|cn, an| [cn, row[an]]}]
+        end
+
+        def record_id(row)
+          row[aliased_primary_key]
+        end
+
+        def instantiate(row)
+          @cached_record[record_id(row)] ||= active_record.send(:instantiate, extract_record(row))
+        end
+      end
+    end
+  end
+end