I have a document that has many embedded documents. I thought it would be faster if when I displayed certain values from the document that I use "only" to limit the attributes retrieved. I noticed that if I excluded the embedded document when using only, the document would automatically be saved when I fetched the parent document. ActiveRecord marks documents as readonly and raises an exception if you try to write to a record that has limited attributes retrieved when using select.

I find this behavior very problematic with Mongoid because it deters the use of larger/complex embedded documents if you can not retrieve specific attributes from a document without having the document automatically updated with default values after retrieval.

I wrote a spec to illustrate what I am talking about.

require 'spec_helper'

class MyDoc
include Mongoid::Document

field :field_1
field :field_2, :default => 'foo'

embeds_one :embedded_doc

after_initialize :set_default_values

def set_default_values

1. It appears that this is the culprit.
2. For some reason it seems that the document is being saved after the next assignment
   self.embedded_doc ||= EmbeddedDoc.new
   end

end

class EmbeddedDoc
include Mongoid::Document

embedded_in :my_doc
field :field_1, :default => 'Bogus'
end

describe MyDoc do

describe "selecting specific fields" do

1. The following spec fails because the embedded document is automatically updated
   it "should result in a readonly record" do
   doc = MyDoc.create(field_1: 'text_1', field_2: 'Correct')
   doc.embedded_doc.field_1.should == 'Bogus'
   doc.embedded_doc.field_1 = 'Howdy'
   doc.save
   MyDoc.first.embedded_doc.field_1.should == 'Howdy'
   MyDoc.count.should == 1
   Rails.logger.debug " – Retrieving specific fields"
   MyDoc.only(:field_1).first
   Rails.logger.debug " – Finished retrieving"
   MyDoc.first.field_2.should == 'Correct'
2. Now notice that the document has been automatically saved
   MyDoc.first.embedded_doc.field_1.should == 'Howdy' # this recieves 'Bogus' and fails
   end

end

end

This is what the log message shows between those two logger statements:

– Retrieving specific fields
MONGODB hub_test['my_docs'].find({}, {:_type=>1, :field_1=>1}).limit(-1).sort([[:_id, :asc]])
MONGODB hub_test['my_docs'].update(

{"_id"=>BSON::ObjectId('4f8720cd4820683c56000004')}

, {"$set"=>{"embedded_doc"=>

{"field_1"=>"Bogus", "_id"=>BSON::ObjectId('4f8720cd4820683c56000006')}

}})
– Finished retrieving

Notice that the document is automatically saved when I call MyDoc.only(:field_1).first