Checkbox check multiple week_days

I am still new to rails/ruby etc and I am trying to build an app that will allow to select work days for a restaurant. The idea is to add an option to select on which days the restaurant works using check_box.

I want to save work days number in the database using their number (0 for sunday etc).

In my app/views/restaurants/_form.html.slim:

 Date::ABBR_DAYNAMES.each do |day|
  = day
  = check_box :work_days_array, {miltiple: true}, Date::ABBR_DAYNAMES.index(day), false

The db/schema.rb contains:

t.integer  "work_days_array",              array: true

It returns the following error:

undefined method `delete' for 0:Fixnum

From what I understand the error is caused by Date::ABBR_DAYNAMES.index(day)

The intended result should be an array of only checked weekdays like

work_days_array: [1, 3, 4]

Can anybody please explain to me the error?