Rails Baffling Result Parsing Array Param Sent Via Curl Multipart POST - ruby-on-rails

Question

Please explain this, or provide a more-correct solution.
class Store < ApplicationRecord
has_and_belongs_to_many :books
end
class Book < ApplicationRecord
has_and_belongs_to_many :stores
end
I have several books in the database. I'd like to create a new store with several books in it.
The following few curl commands result in Rails parsing an empty array for params[book_ids]:
curl -F "author[book_ids[]]=[1,2]" -F "author[description]=some description" localhost:3000/authors.multipart
Started POST "/authors.multipart" for 127.0.0.1 at ...
Processing by AuthorsController#create as MULTIPART
Parameters: {"author"=>{"gesture_ids"=>[], "description"=>"some description", #headers ...
curl -F "author[book_ids[]]=\"[1,2]\"" -F "author[description]=some description" localhost:3000/authors.multipart
Started POST "/authors.multipart" for 127.0.0.1 at ...
Processing by AuthorsController#create as MULTIPART
Parameters: {"author"=>{"gesture_ids"=>[], "description"=>"some description", #headers ...
curl -F "author[book_ids[]]=1" -F "author[book_ids[]]=2" -F "author[description]=some description" localhost:3000/authors.multipart
Started POST "/authors.multipart" for 127.0.0.1 at ...
Processing by AuthorsController#create as MULTIPART
Parameters: {"author"=>{"gesture_ids"=>[], "description"=>"some description", #headers ...
But the following command does the trick:
curl -F "author[book_ids[]=1" -F "author[book_ids[]=2" -F "author[description]=some description" localhost:3000/authors.multipart
Started POST "/authors.multipart" for 127.0.0.1 at ...
Processing by AuthorsController#create as MULTIPART
Parameters: {"author"=>{"gesture_ids"=>["1","2"], "description"=>"some description", #headers ...
As you can see, the command is a missing a right brace in a couple of places, but it works. Checking the trace, curl is indeed sending the following for the first book.
0000: HTTP/1.1 100 Continue
=> Send data, 502 bytes (0x1f6)
0000: --------------------------538dd4bc2333141c
002c: Content-Disposition: form-data; name="store[book_ids[]"
0068:
006a: 1
006f: --------------------------538dd4bc2333141c

Answer 1

The correct syntax of author[book_ids][]=1
The syntax in this case is wrapper[key] = value (like you have for description) so for an Array we state that it is an Array of values wrapper[key][] then the = appends the value to the Array.
Write now author[book_ids[]]=[1,2] would technically be parsed as author: {book_ids: []} = [1,2] which is clearly incorrect and the parser just ignores the assignment.