Active Record Association Collection
CollectionAssociation: HasManyAssociation => has_many HasManyThroughAssociation + ThroughAssociation => has_many :through
CollectionAssociation class provides common methods to the collections defined by has_and_belongs_to_many, has_many or has_many with +:through association+ option.
You need to be careful with assumptions regarding the target: The proxy does not fetch records from the database until it needs them, but new ones created with build are added to the target. So, the target may be non-empty and still lack children waiting to be read from the database. If you look directly to the database you cannot assume that’s the entire collection because new records may have been added to the target, etc.