How to create a select like in Avito?

Question

Ruby on Rails

So that both the category and subcategory are displayed, but that the category is disabled

Answer 1

Read here

grouped_options = [
  ['Group 1',
    ["Item 1", "Item 2", "Item 3"]],
  ['Group 2',
    ["Item 1", "Item 2", "Item 3", "Item 4"]]
]
grouped_options_for_select(grouped_options)

Answer 2

Select2