module Sequel
  # Empty namespace that plugins should use to store themselves,
  # so they can be loaded via Model.is.
  #
  # Plugins should be modules with one of the following conditions:
  # * A singleton method named apply, which takes a model and 
  #   additional arguments.
  # * A module inside the plugin module named InstanceMethods,
  #   which will be included in the model class.
  # * A module inside the plugin module named ClassMethods,
  #   which will extend the model class.
  # * A module inside the plugin module named DatasetMethods,
  #   which will extend the model's dataset.
  module Plugins
  end
  
  class Model
    # Loads a plugin for use with the model class, passing optional arguments
    # to the plugin. If the plugin has a DatasetMethods module and the model
    # doesn't have a dataset, raise an Error.
    def self.is(plugin, *args)
      m = plugin_module(plugin)
      raise(Error, "Plugin cannot be applied because the model class has no dataset") if m.const_defined?("DatasetMethods") && !@dataset
      if m.respond_to?(:apply)
        m.apply(self, *args)
      end
      if m.const_defined?("InstanceMethods")
        class_def(:"#{plugin}_opts") {args.first}
        include(m::InstanceMethods)
      end
      if m.const_defined?("ClassMethods")
        meta_def(:"#{plugin}_opts") {args.first}
        extend(m::ClassMethods)
      end
      if m.const_defined?("DatasetMethods")
        dataset.meta_def(:"#{plugin}_opts") {args.first}
        dataset.metaclass.send(:include, m::DatasetMethods)
        def_dataset_method(*m::DatasetMethods.instance_methods)
      end
    end
    metaalias :is_a, :is
  
    ### Private Class Methods ###

    # Returns the gem name for the given plugin.
    def self.plugin_gem(plugin) # :nodoc:
      "sequel_#{plugin}"
    end

    # Returns the module for the specified plugin. If the module is not 
    # defined, the corresponding plugin gem is automatically loaded.
    def self.plugin_module(plugin) # :nodoc:
      module_name = plugin.to_s.gsub(/(^|_)(.)/){|x| x[-1..-1].upcase}
      if not Sequel::Plugins.const_defined?(module_name)
        require plugin_gem(plugin)
      end
      Sequel::Plugins.const_get(module_name)
    end

    private_class_method :plugin_gem, :plugin_module
  end
end