Mongoid 8.0.8 and 8.1.5 are the latest patch releases in the 8.0 and 8.1 series. They include the following two fixes:
MONGOID-5704: By default, associations (like belongs_to) are validated when a document is saved. However, Mongoid was aggressively loading persisted associations and validating them, which led to a significant performance regression in Mongoid 8+ (versus Mongoid 7.x). This patch fixes this regression by only validating associations that are (1) currently in-memory, and (2) either unpersisted or modified.
MONGOID-5709: has_and_belongs_to_many associations on embedded docments were broken, and attempting to use them would result in an “InvalidPath” exception. This patch release fixes that bug.