Description

Mongoid 9.1.0 raises NoMethodError when a has_and_belongs_to_many association stores an empty foreign-key array and the association is reset before it is enumerated. Mongoid 9.0.9 does not reproduce the issue.

Mongoid itself can create this data. For example, Team.create!(user_ids: []) stores an explicit empty array.

Reproduction

class User
  include Mongoid::Document
end

class Team
  include Mongoid::Document
  has_and_belongs_to_many :users
end

team = Team.create!(user_ids: [])
team.reload

users = team.users
users.reset
users.empty?
users.size
users.to_a

On Mongoid 9.1.0, calls after reset raise NoMethodError. For example, to_a raises:

NoMethodError: undefined method `selector` for nil

Expected behaviour

An explicitly stored empty foreign-key array should represent an empty association. After reset, empty?, size, and enumeration should return the empty result without querying the database.

Actual behaviour

HasAndBelongsToMany::Buildable#build treats an empty array as {{ $lookup }} output because {{[].all?

}} returns true. It returns a plain Array, so HasMany::Enumerable has no unloaded criteria. After reset, empty?, size, or enumeration can call methods on the missing criteria object.

Root cause

The array-of-hashes {{ $lookup }} branch was added by MONGOID-5731 in pull request 6081. Mongoid 9.0.9 did not have this branch, so the empty array reached query_criteria instead.

The same Array target issue also exists for some {{ $lookup }} results, including non-empty has_and_belongs_to_many results and empty has_many results. Those cases predate this regression and are outside this fix. They should be addressed separately in HasMany::Enumerable.

Related Mongoid tickets

MONGOID-5030 covers short-circuiting queries whose {{ $in }} value is an empty array. This issue needs an association-specific none criteria because the target's empty? and size methods must avoid a query even when the general short-circuit option is disabled. Proxy count and exists? are outside this guarantee because they build criteria from the stored IDs.
MONGOID-6000 (recreated from this)

Suggested fix

The {{ $lookup }} branch should accept only non-empty arrays of hashes. For an empty foreign-key array, the implementation should pass nil to query_criteria, which returns crit.none. This preserves a criteria-backed target and avoids database queries from the target's empty?, size, and enumeration methods.

The existing eager-load specs cover the includes/preload path, including empty results without an extra query. The persisted empty-array path needs a separate regression test because it reaches Buildable#build through a different route.

Environment

Mongoid 9.1.0

The application stores explicit empty foreign-key arrays in its association data.