metadata_validations

Validations for Rails models with serialized metadata stored in the database. Provides a convenient way of validating required and optional fields.

Migrations

If your model doesn't already have a metadata column, create one (the column name is arbitrary):

class AddMetaDataToAssets < ActiveRecord::Migration
  def self.up
    add_column :assets, :meta, :text
  end

  def self.down
    remove_column :assets, :meta
  end
end

Metadata columns are required to be text fields. All metadata is stored as a hash with symbolized keys.

Models

To associate and validate metadata, let's take a look at the model:

class Asset < ActiveRecord::Base

  has_metadata :meta, :required_fields => [ :name, :type ], :optional_fields => [ :author ]

  # ...

end

metadata_validations provide a single new class-level method, has_metadata. The first argument is the name of the column. You can use has_metadata without specifying required_fields or optional_fields. The plugin will automatically ensure that default values are specified and will take care of casting keys to symbols.

Accessors

You also get access to meta_accessor which will dynamically define methods for you on the model. For example:

class Asset < ActiveRecord::Base

  has_metadata :meta, :required_fields => [ :name, :type ], :optional_fields => [ :author ]
  meta_accessor :meta, :name

  # ...

end

Now you'll have access to @asset.name and @asset.name=, which is convenient for FormHelpers. If your metadata fields have accessors, they can easily be used with form helpers:

<% form_for @asset do |f| %>
  <%= f.text_field :name %>
<% end %>

(Note that the first parameter to meta_accessor is the serialized column's name.)

Advanced Validations

The has_metadata method can also take a proc so that the fields are evaluated at run time. The proc should totally return an array of symbols. This is especially useful if you have polymorphic models or intend to require different fields in different situations.

Let's take a look at a real-world example:

class Video < ActiveRecord::Base

  belongs_to :video_type

  has_metadata :meta, :required_fields => Proc.new { |v| v.video_type.blank? ? [] : v.video_type.required_fields }

  # ...

end

Each Video belongs_to a VideoType and the database contains each video type's required fields. In this example, the proc also allows for videos without a VideoType (in which case there are no required fields).

The Future

I could see adding: metadata form helpers and magic fields.

Copyright (c) 2009 Chris Bielinski chris@shadowreactor.com, released under the MIT license.