(Originally reported at https://github.com/mongodb/mongoid/discussions/5674)

$addToSet is weird.

Here is the model:

class BuggyModel
  include ::Mongoid::Document
  store_in collection: 'buggy_models'

  field :list, type: :array, default: []
end

Create one element:

::BuggyModel.create!({ _id: 'test' })

Add something to the set:

::BuggyModel.update_all({
  '$addToSet': {
    list: 'a',
  },
})

Reload:

::BuggyModel.find('test').list

Expecting: ['a']
Actual: [nil]

Why?

Digging further, the problem is in mongoize of Array fields that was called from hash._consolidate_.

The workaround exists by omitting the type of the field. Changing this:

field :list, type: :array, default: []

To this:

field :list, default: []

Probably Mongoid::Persistable::LIST_OPERATIONS handling needs revises.