Select Some Columns

quick_bulk_select.rb

# 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