在单独的表中与 id 和类型的多态关联

Question

I have a polymorphic relation in my application for custom fields similar to this:

class CustomFieldValue < ApplicationRecord
  belongs_to :custom_field_type
  belongs_to :custom_field_valuable, polymorphic: true
end

class CustomFieldType < ApplicationRecord
  has_many :custom_field_values
end

Basically the custom_field_type table holds a few details about the field type (selection, checkbox etc) as well as a 'model_type' column for the model the custom field belongs to.

The custom_field_value table has the expected polymorphic columns 'custom_field_valuable_type' and 'custom_field_valuable_id' which works fine but seeing as custom_field_value belongs to custom_field_type which already has the model_type stored, I was wondering if there was a way I can do away with the custom_field_valuable_type?

I've tried redefining 'custom_field_valuable_type' with

def custom_field_valuabkle_type
  self.custom_field_type.model_type
end

But that doesn't work as there is no column to query.

Answer 1

I was wondering if there was a way I can do away with the custom_field_valuable_type?

No. Polymorphic associations expect a association_name_type column to exist on the table with the association. Putting the type column another table is not possible AFAIK and would be really over-complicated. Polymorphic associations are already a hacky solution to the Object-relational impedance mismatch problem and you're going from saying that the foreign key is constructed from two columns to the foreign key is constructed of from columns on different tables! That doesn't improve anything and just increases the mismatch. Your approach is not going to work as the join queries are done in the database and don't call your getter method.

I think you're also confusing two completely different concepts. What you are doing is just a variation on the Entity Attribute Value (EAV) pattern. What your CustomFieldType provides to the setup is normalization of the attributes - so that your not duplicating the definition of attributes with each value. This has nothing to do with linking Value -> Entity.