Rails checkbox not saving any value

I am new to Rails and I have these checkboxes that display the options just fine, but aren't changing anything in the database as the form is submitted. The form in views has the following piece of code:

<%= form_for(@sector) do |f| %>
  <%= f.collection_check_boxes :admins_id, Admin.all, :id, :name %>
<% end %>

and this is the corresponding action in the sectors controller:

def update
  @sector = Sector.find(params[:id])
  @sector.admins_id = params[:admins_id]

  respond_to do |format|
    if @sector.update(sector_params)
      format.html { redirect_to @sector, notice: 'Sector was successfully updated.' }
      format.json { render :show, status: :ok, location: @sector }
    else
      format.html { render :edit }
      format.json { render json: @sector.errors, status: :unprocessable_entity }
    end
  end
end
private
  def sector_params
    params.require(:sector).permit(:title, :admins_id)
  end

And, finally, I have these relations in the models:

class Sector < ActiveRecord::Base
  has_many :admins, dependent: :destroy
  validates :title, presence: true
  validates :title, uniqueness: true
end
class Admin < ActiveRecord::Base
  belongs_to :sector
end

Also, I can create and assign admins just fine in the rails console.