为什么此多态关联的source_type始终为0？

Question

For some reason the source type for a polymorphic has_many :through association is always 0, despite having set a :source_type .

Here's what my models look like...

Foo:

has_many :tagged_items, :as => :taggable
has_many :tags, :through => :tagged_items

Bar:

has_many :tagged_items, :as => :taggable
has_many :tags, :through => :tagged_items

TaggedItem:

belongs_to :tag
belongs_to :taggable, :polymorphic => true

Tag:

has_many :tagged_items
has_many :foos, :through => :tagged_items, :source => :taggable, :source_type => "Foo"
has_many :bars, :through => :tagged_items, :source => :taggable, :source_type => "Bar"

As near as I can tell that's a totally fine setup, and I am able to create / add tags, but the taggable_type always ends up being 0.

Any idea's why? Google has turned up nothing.

Answer 1

A working example with tests for the models in the question is found here .

The reason why it didn't work on the question is the migration db/migrate/[timestamp]_create_tagged_items should be generated like this:

class CreateTaggedItems < ActiveRecord::Migration
  def change
    create_table :tagged_items do |t|
      t.belongs_to :tag, index: true
      t.references :taggable, polymorphic: true

      t.timestamps
    end
  end
end

Note that t.references :taggable, polymorphic:true will generate two columns on schema.rb :

t.integer  "taggable_id"
t.string   "taggable_type"

So, with the same models in the question and this migration, the following tests pass:

 require 'test_helper'

 class TaggedItemTest < ActiveSupport::TestCase
  def setup
    @tag = tags(:one)
  end

  test "TaggedItems have a taggable_type for Foo" do
    foo = Foo.create(name: "my Foo")
    @tag.foos << foo

    assert TaggedItem.find(foo.id).taggable_type == "Foo"
  end


  test "TaggedItems have a taggable_type for Bar" do
    bar = Bar.create(name: "my Bar")
    @tag.bars << bar

    assert TaggedItem.find(bar.id).taggable_type == "Bar"
  end
end

---

Note: Rails 3 has an active issue about has_many :through and polymorphic associations, as shown here . Though, in Rails 4, this is solved.

PS: Since I did some research on this question, I might as well post the answer in case someone can benefit from it... :)

Answer 2

Figured this one out myself, just answering because I'm sure I'm not the first or last person to make this stupid mistake (in fact I may have done it before). I put the column type on the taggable_type field as an integer instead of a string.

You'd think this might cause an error, but it doesn't. It just doesn't work.