tomlea · January 6, 2012 12:06 · Jan 6, 2012
diff --git a/quick_bulk_select.rb b/quick_bulk_select.rb
@@ -0,0 +1,86 @@
+# Introduction:
+
+We need to select large numbers of items (300k or more), based on active record associations (and selectors, and all that faff), but we only care about the ids.
+
+This is easily solved by bla_bla_bla_scope.map(&:id). This however takes about 16s of CPU grinding ruby to get to the final answer via AR objects. AR objects that contain only an ID.
+
+Why not just use the SQL generated by the scopes and other voodoo to just pump the SQL through? That's what this does, in the most complex way imaginable!
+
+# Usage:
+
+Person.ids # => all the person ids. (or whatever the key field is).
+
+Person.order("name").select_name_and_id #=> Sorted names,id pairs.
+
+Person.first.friends.order("name").select_name_and_id #=> Sorted names,id pairs of the first person's friends.
+
+#Initializer:
+module ActiveRecord
+  class DynamicSelectMatch
+    def self.match(method)
+      return unless method.to_s =~ /^select_([_a-zA-Z]\w*)$/
+      new($1 && $1.split('_and_'))
+    end
+
+    def initialize(attribute_names)
+      @multi           = attribute_names.size > 1
+      @attribute_names = attribute_names
+    end
+
+    attr_reader :multi, :attribute_names
+    alias :multi? :multi
+  end
+
+
+  module SelectJustSomeColumns
+    def select_column_values(*columns)
+      columns = columns.map{|c| "#{quoted_table_name}.#{connection.quote_column_name(c)}" }
+      connection.select_rows(scoped(:select => columns.join(", ")).arel.to_sql )
+    end
+
+    def select_column_value(column)
+      column = "#{quoted_table_name}.#{connection.quote_column_name(column)}"
+      connection.select_values(scoped(:select => column).arel.to_sql )
+    end
+
+    def ids
+      select_column_value(primary_key)
+    end
+
+    def method_missing(method_id, *arguments, &block)
+      if match = DynamicSelectMatch.match(method_id)
+        super unless all_attributes_exists?(match.attribute_names)
+
+        if match.multi?
+          self.class_eval <<-METHOD, __FILE__, __LINE__ + 1
+            def self.#{method_id}                                        # def self.select_user_name_and_password
+              select_column_values(:#{match.attribute_names.join(',:')}) #   select_column_values(:user_name, :password)
+            end                                                          # end
+          METHOD
+        else
+          col_name = match.attribute_names.first.to_sym.inspect
+          self.class_eval <<-METHOD, __FILE__, __LINE__ + 1
+            def self.#{method_id}                                        # def self.select_user_name
+              select_column_value(#{col_name})                           #   select_column_values(:user_name)
+            end                                                          # end
+          METHOD
+        end
+
+        send(method_id, *arguments)
+      else
+        super
+      end
+    end
+
+    def respond_to?(method_id, include_private = false)
+      if match = DynamicSelectMatch.match(method_id)
+        return true if all_attributes_exists?(match.attribute_names)
+      end
+
+      super
+    end
+
+  end
+
+  Base.extend(SelectJustSomeColumns)
+end