def relation(name)
      if @relations.key? name
        Relation.new @context, @relations.fetch(name)
      else
        Relation.new @context, @relations.fetch(name.to_s)
      end
    end
def relation(name)
      if @relations.key? name
        Relation.new @context, @relations.fetch(name)
      else
        Relation.new @context, @relations.fetch(name.to_s)
      end
    end
Return relation with given name.

 @param name [String, Symbol] Relation name.
 @return [Relation] Relation.