I have a many-to-many relationship between a model User and Picture. These are linked by a join table called Picturization.
If I obtain a list of users of a single pict
Have an additional column something like sequence in picturization table and define sort order as default scope in your Picturization
default_scope :order => 'sequence ASC'
If you want default sort order based on modified_at then use following default scope
default_scope :order => 'modified_at DESC'