I've found it - it was because of quirky way I was creating checkboxes for HABTM
= check_box_tag "user_setting[district_ids][#{district.id}]", district.id, user.settings.district_ids.include?(district.id)
= label_tag "user_setting[district_ids][#{district.id}]", district.name
For no particular reason I've inserted ids into params keys AND values. And because of that those were passed to params
object as hash. In test though those were sent as array. So it was the view to blame.
= check_box_tag "user_setting[district_ids][]", district.id, user.settings.district_ids.include?(district.id)
= label_tag "user_setting[district_ids][]", district.name