```ruby
module ModelPresenter
  module HasMany
    # The DSL adds an instance method `relation` to represnet a `has_many` relationship
    #
    #
    #     class User
    #       include ModelPresenter::Base
    #       has_many :groups, presenter_class: Presenters::Group
    #     end
    #
    # The macro will generates a ```groups``` methods, which will return an array. Each element of the array is an instance of ```Presenters::Group``` whose ```model``` is one of the group models that the user has.
    #
    # @param [Symbol, #read] relation the name of the relationship
    # @param [Hash, #read] options Currently it accepts one key `presenter_class` is a class. The class is supposed to be a presenter class. The `relation` method will return an array of elements that are instances of the class
    #
    # @return none
    def has_many(relation, options)
      self.send :define_method, relation do
        instance_variable_name = "@#{relation}"
        instance_variable = instance_variable_get("@#{relation}")
        return instance_variable if instance_variable
        instance_variable_set(
          instance_variable_name,
          model.send(relation).map do |relation_model|
            options[:presenter_class].new relation_model
          end
        )
      end
    end
  end
end

```

Use rdoc format for inline code
user
```ruby
module ModelPresenter
  module HasMany
    # The DSL adds an instance method +relation+ to represnet a +has_many+ relationship
    #
    #
    #     class User
    #       include ModelPresenter::Base
    #       has_many :groups, presenter_class: Presenters::Group
    #     end
    #
    # The macro will generates a +groups+ methods, which will return an array. Each element of the array is an instance of +Presenters::Group+ whose +model+ is one of the group models that the user has.
    #
    # @param [Symbol, #read] relation the name of the relationship
    # @param [Hash, #read] options Currently it accepts one key +presenter_class+ is a class. The class is supposed to be a presenter class. The +relation+ method will return an array of elements that are instances of the class
    #
    # @return none
    def has_many(relation, options)
      self.send :define_method, relation do
        instance_variable_name = "@#{relation}"
        instance_variable = instance_variable_get("@#{relation}")
        return instance_variable if instance_variable
        instance_variable_set(
          instance_variable_name,
          model.send(relation).map do |relation_model|
            options[:presenter_class].new relation_model
          end
        )
      end
    end
  end
end

```
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