module Piglet
  module Relation
    class Join # :nodoc:
      include Relation
    
      def initialize(relation, description)
        @join_fields = Hash[*description.select { |k, v| k.is_a?(Relation) }.flatten]
        @sources = @join_fields.keys
        @using = description[:using]
        @parallel = description[:parallel]
      end
      
      def schema
        schemas = @sources.map { |s| s.schema }
        schemas.first.union(schemas[1..-1])
      end
    
      def to_s
        joins = @sources.map { |s| "#{s.alias} BY #{@join_fields[s]}" }.join(', ')
        str  = "JOIN #{joins}"
        str << " USING \"#{@using.to_s}\"" if @using
        str << " PARALLEL #{@parallel}" if @parallel
        str
      end
    end
  end
end