how to permit an array with strong parameters

This https://github.com/rails/strong_parameters seems like the relevant section of the docs:

The permitted scalar types are String, Symbol, NilClass, Numeric, TrueClass, FalseClass, Date, Time, DateTime, StringIO, IO, ActionDispatch::Http::UploadedFile and Rack::Test::UploadedFile.

To declare that the value in params must be an array of permitted scalar values map the key to an empty array:

params.permit(:id => [])

In my app, the category_ids are passed to the create action in an array

"category_ids"=>["", "2"],

Therefore, when declaring strong parameters, I explicitly set category_ids to be an array

params.require(:question).permit(:question_details, :question_content, :user_id, :accepted_answer_id, :province_id, :city, :category_ids => [])

Works perfectly now!

(IMPORTANT: As @Lenart notes in the comments, the array declarations must be at the end of the attributes list, otherwise you'll get a syntax error.)

If you want to permit an array of hashes(or an array of objects from the perspective of JSON)

params.permit(:foo, array: [:key1, :key2])

2 points to notice here:

1. array should be the last argument of the permit method.
2. you should specify keys of the hash in the array, otherwise you will get an error Unpermitted parameter: array, which is very difficult to debug in this case.

It should be like

params.permit(:id => [])

Also since rails version 4+ you can use:

params.permit(id: [])