Updating embedded document with Mongoid updates the parent document instead

Question

When I try to update an attribute on an embedded document (embeds_many), mongoid fails to save the changes, and curiously adds the changed attribute as a new attribute on the parent document instead. Here is a simple unit test that illustrates what I am trying to do:

class Tab
  include Mongoid::Document
  field :name, :type => String
  embeds_many :components, :class_name => 'TabComponent'
end

class TabComponent
  include Mongoid::Document
  embeds_many :components, :class_name => "TabComponent"
end

class TabColumn < TabComponent
  field :width, :type => Integer
end

require 'test_helper'

class TabTest < ActiveSupport::TestCase
  test "create new tab" do
    tab = Tab.new({
      :name => "My Demo Tab",
      :components => [TabColumn.new({
        :width => 200
      })]
    })

    tab.save!

    tab.components[0].width = 300
    tab.save!

    assert_equal tab.components[0].width, 300 # passes
    tab.reload
    assert_equal tab.components[0].width, 300 # fails!
  end
end

Here is the log output:

MONGODB (39ms) beam_test['system.namespaces'].find({})
MONGODB (27ms) beam_test['$cmd'].find({"count"=>"tabs", "query"=>{}, "fields"=>nil}).limit(-1)
MONGODB (38ms) beam_test['tabs'].find({})
MONGODB (0ms) beam_test['tabs'].remove({:_id=>BSON::ObjectId('4fb153c4c7597fbdac000002')})
MONGODB (0ms) beam_test['tabs'].insert([{"_id"=>BSON::ObjectId('4fb15404c7597fccb4000002'), "name"=>"My Demo Tab", "components"=>[{"_id"=>BSON::ObjectId('4fb15404c7597fccb4000001'), "_type"=>"TabColumn", "width"=>200}]}])
MONGODB (0ms) beam_test['tabs'].update({"_id"=>BSON::ObjectId('4fb15404c7597fccb4000002')}, {"$set"=>{"width"=>300}})
MONGODB (27ms) beam_test['tabs'].find({:_id=>BSON::ObjectId('4fb15404c7597fccb4000002')}).limit(-1)

Am I doing something wrong? Note that I don't think the problem is polymorphism, if I simplify things by putting width on TabComponent, the same behavior is observed.

Answer 1

You have a simple error in your relation, instead use the following to complete the symmetry of your embeds_many / embedded-in relation.

class TabComponent
  include Mongoid::Document
  embedded_in :tab
end

In your log output above, you see:

MONGODB (0ms) beam_test['tabs'].update({"_id"=>BSON::ObjectId('4fb15404c7597fccb4000002')}, {"$set"=>{"width"=>300}})

After the above fix, I now get:

MONGODB (0ms) free11819_mongoid_embedded_update_test['tabs'].update({"_id"=>BSON::ObjectId('4fb270fee4d30bbc20000002')}, {"$set"=>{"components.0.width"=>300}})

Note the difference width versus components.0.width.

Hope that this helps to get you on your way.