Can't modify frozen String on Hash - ruby-on-rails

Question

I am a bit confused with the frozen string and utilizing them with test cases.
I just added the following line at the top of my test cases :
# frozen_string_literal: true
And i have the following two test cases:
test "Create upload invoice invalid invoice id" do
post :upload, params = {invoices_data: [{invoice_id: 987654, unit_id: 1321}]}
assert_response :not_found
end
test "Create upload invoice request to fortnox with non array request parameter" do
request = {invoices_data: {invoice_id: "invoice.id", unit_id: 321}}
post :upload_invoices, params = request
assert_response :bad_request
end
All of a sudden my second test failed with
RuntimeError: can't modify frozen String
at this line
post :upload_invoices, params = request
however, if I change the string to some symbol for instance :invoice_id then it works just fine.
Can someone guide why about the following two things:
Why does sending a string value fails in this case reporting that I
am trying to modify a String and which string value I am trying to
modify?
Why does it fail on post request, if it has to fail then it
should fail when creating the request i.e request = {invoices_data: {invoice_id: "invoice.id", unit_id: 321}}
What i can do to send string value instead of Symbol in the hash?

Answer 1

1a) Sending a string value fails in this case because your upload_invoices controller action attempts to modify the invoice_id parameter itself. (Or you're running an old version of Rails where the #post method itself attempts to modify the invoice_id parameter by converting it to UTF-8 encoding.)
1b) The string value you're trying to modify is "invoice.id".
2 ) It fails on the post request and not the assignment to the request variable because the assignment to the request variable is not where the attempted modification happens. The frozen string literal is attempted to be modified by the call to #post. See answer 1a above.
3 ) You can send a non-frozen string value in the hash a few different ways. You could remove the # frozen_string_literal: true magic comment, but I feel you don't want to do that. Otherwise, the simplest thing to do is to send along a duplicate of the string with either +'invoice.id' or the less esoteric 'invoice.id'.dup. Or you can create a non-literal string with something silly like ['invoice', 'id'].join('.') or :invoice.to_s. No doubt there are other ways.
However, it seems EXTREMELY unlikely you want to pass a string here at all. The invoice_id parameter is almost assuredly an integer, and passing a string to it makes little sense unless I guess you're trying to test that the controller action can handle that kind of erroneous input. If so, one of the string duplication techniques +'string_literal'/'string_literal'.dup would be your best option.
I would wager by the name of the test that you're actually trying to send along a real invoice_id which means you don't want to pass along a string, but instead an integer. Maybe the ID of an Invoice fixture you have setup?
And on another slightly unrelated note, you're not passing params to the #post method properly. It should be params: ... not params = ....