After updating to Mongoid 8 from 7.5 we noticed 2 problems with `belongs_to` associations:

• performance issues
• validation issues

tl;dr - Mongoid 8 started to validate full chain of dependent objects.

Here is simplified example (our real app scenario is way more complex):

# MODELS

class A
  include Mongoid::Document
  include Mongoid::Timestamps

  belongs_to :b
end

class B
  include Mongoid::Document
  include Mongoid::Timestamps

  has_one :a
  belongs_to :c
end

class C
  include Mongoid::Document
  include Mongoid::Timestamps

  has_one :b
  embeds_many :embs
end

class Emb
  include Mongoid::Document
  include Mongoid::Timestamps

  embedded_in :c
  belongs_to :d
end

class D
  include Mongoid::Document
  include Mongoid::Timestamps

  has_one :emb

  field :name, type: String
  validates :name, presence: true
end 

# SETUP

ds = 20.times.map { D.create!(name: 'abc') }
embs = ds.map { |d| Emb.new(d:) }
c = C.create!(embs:)
b = B.create!(c:) 

Performance issue

# TEST SCENARIO

b = B.last
a = A.create!(b:) 

In Mongoid 7.5 you'll see only 2 mongo queries:

| STARTED | {"find"=>"bs", ...
| STARTED | {"insert"=>"as", ... 

However in Mongoid 8.x it fetches the full chain of dependent objects which basically makes create action X times slower (depending on number of associations):

| STARTED | {"find"=>"bs", ...
| STARTED | {"find"=>"cs", ...
| STARTED | {"find"=>"ds", ... x20
| STARTED | {"insert"=>"as", ... 

This approach looks incorrect. It's ok to validate 1st level associations but validating the whole chain could lead to a problems. Besides performance issues there is a chance that some record deep below has some validation errors which will lead to impossibility to save.

Validation issue

This works in 7.5, but doesn't work in 8:

b = B.last
b.c.embs.first.d.set(name: nil) # make record invalid
exit

b = B.last
a = A.create!(b:) 

Mongoid 8 will show:

/Users/xxx/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/mongoid-8.1.3/lib/mongoid/persistable.rb:125:in `fail_due_to_validation!':  (Mongoid::Errors::Validations)
message:
  Validation of A failed.
summary:
  The following errors were found: B is invalid
resolution:
  Try persisting the document with valid data or remove the validations.
 

While Mongoid 7.5 will correctly create a record.

It's a pretty common scenario when some record which was created N years ago is invalid. And that's fine. We are not touching or updating it. But this creates issues in places where you don't expect them. Certain parts of application start to trigger validation errors which are not related to the source of call.

And obviously this happens when updating record and by just calling valid?.

Additional notes:

• Mongoid.belongs_to_required_by_default is true

• all belongs_to assiciations are required and we can't change any of these options.