Question

You know how some bug trackers (and other software) allow you to add custom fields?

Typically this is done with a data structure that looks something like this:

  Items
----------
 ID | NAME | ITEM_TYPE_ID


 FieldDefinitions
---------------------------------------
 ID | ITEM_TYPE_ID | FIELD_NAME | FIELD_TYPE

 FieldValues
---------------------------------------
ID | FIELD_ID | ITEM_ID | VALUE

I’m trying to figure out the best way to approach this design in Rails. There will be many models which I want to allow extending simple properties.

When I retreive an Item I’d like it to include a hash of the addition field values that have been defined for that model.

Answer 1

Something like…?

class Item < ActiveRecord::Base
  has_many :field_values
  has_many :field_definitions, :through => :field_values

  def custom_fields_hash
    cfh = {}
    self.field_values.each |fv|
      cfh[fv.field_definition] = fv
    end
    cfh
  end
end

class FieldValue < ActiveRecord::Base
  belongs_to :item
  belongs_to :field_definition
end

class FieldDefinition < ActiveRecord::Base
  has_many :field_values
  has_many :items, :through => field_values
end

Or, you could change

cfh[fv.field_definition] = fv

…to…

cfh[fv.field_definition.field_name] = fv.value