Rails 5: Nested attributes aren't updated for model - ruby-on-rails

Question

I can't get rails to update my nested attributes, though regular attributes work fine. This is my structure:
unit.rb:
class Unit < ApplicationRecord
has_many :unit_skill_lists
has_many :skill_lists, through: :unit_skill_lists, inverse_of: :units, autosave: true
accepts_nested_attributes_for :skill_lists, reject_if: :all_blank, allow_destroy: true
end
unit_skill_list.rb:
class UnitSkillList < ApplicationRecord
belongs_to :unit
belongs_to :skill_list
end
skill_list.rb:
class SkillList < ApplicationRecord
has_many :unit_skill_lists
has_many :units, through: :unit_skill_lists, inverse_of: :skill_lists
end
And this is (part of) the controller:
class UnitsController < ApplicationController
def update
#unit = Unit.find(params[:id])
if #unit.update(unit_params)
redirect_to edit_unit_path(#unit), notice: "Unit updated"
else
redirect_to edit_unit_path(#unit), alert: "Unit update failed"
end
end
private
def unit_params
unit_params = params.require(:unit).permit(
...
skill_list_attributes: [:id, :name, :_destroy]
)
unit_params
end
end
The relevant rows in the form (using formtastic and cocoon):
<%= label_tag :skill_lists %>
<%= f.input :skill_lists, :as => :check_boxes, collection: SkillList.where(skill_list_type: :base), class: "inline" %>
Any idea where I'm going wrong? I have tried following all guides I could find but updating does nothing for the nested attributes.
Edit after help from Vasilisa:
This is the error when I try to update a Unit:
ActiveRecord::RecordInvalid (Validation failed: Database must exist):
This is the full unit_skill_list.rb:
class UnitSkillList < ApplicationRecord
belongs_to :unit
belongs_to :skill_list
belongs_to :database
end
There is no input field for "database". It is supposed to be set from a session variable when the unit is updated.

Answer 1

If you look at the server log you'll see something like skill_list_ids: [] in params hash. You don't need accepts_nested_attributes_for :skill_lists, since you don't create new SkillList on Unit create/update. Change permitted params to:
def unit_params
params.require(:unit).permit(
...
skill_list_ids: []
)
end
UPDATE
I think the best options here is to set optional parameter - belongs_to :database, optional: true. And update it in the controller manually.
def update
#unit = Unit.find(params[:id])
if #unit.update(unit_params)
#unit.skill_lists.update_all(database: session[:database])
redirect_to edit_unit_path(#unit), notice: "Unit updated"
else
redirect_to edit_unit_path(#unit), alert: "Unit update failed"
end
end