class Image < ActiveRecord::Base
  has_many :custom_fields
  # has a name column
end

class CustomField < ActiveRecord::Base
  belongs_to :field, :polymorphic => true
  # has only IDs
end

class StringField < ActiveRecord::Base
  has_one :custom_field, :as => :field
  # has a value column
end

Image.find(:join => {:custom_fields=>:field}, :conditions => {"custom_fields.name"=>name, "custom_fields.field.value"=>value})