Rails strange parameter added on POST request

I realized recently that a parameter is added when I try to create multiple items of my class User in a POST request.

I have some permitted parameters:

def user_params
  params.permit(:account_id, users: [:type, :name, :account_id])
end

Here is my create method in my UsersController:

def create
  if user_params[:users].blank?
       render status: :no_content
       return
  end

  begin
    _users= User.create!(user_params[:users])
    render json: _users, status: :created
  rescue ActiveRecord::RecordInvalid
    render json: _users.errors, status: :unprocessable_entity
  end
end

When I try to test this using Postman, I use these parameters:

{
    "users": [{
        "type": "test", 
        "name": "test", 
        "account_id": "2DAC41seuRwGIxTn4PQPZc"
    }, {
        "type": "test2", 
        "name": "test2", 
        "account_id": "2DAC41seuRwGIxTn4PQPZc"
    }]
}

Here is also the router conf:

resources :account, except: [:index] do
  resources :users, only: [:create, :show, :index]
end

Each time I do a POST request on /accounts/2DAC41seuRwGIxTn4PQPZc/users, I see in the rails console that the parameters are:

Parameters: {"users"=>[{"type"=>"test", "name"=>"test", "account_id"=>"2DAC41seuRwGIxTn4PQPZc"}, {"type"=>"test2", "name"=>"test2", "account_id"=>"2DAC41seuRwGIxTn4PQPZc", }], "account_id"=>"2DAC41seuRwGIxTn4PQPZc", "user"=>{}}

Unpermitted parameter: :user.

I don't know where this parameter come from. Can someone enlighten me ?

Note that I'm still learning rails and ruby overall, maybe I'm missing something obvious.

Solution

As @spickerman mentioned it, the problem was that the ParamsWrapper was expecting a JSON format, and according to the doc:

If you enable ParamsWrapper for :json format, [...], it will be wrapped into a nested hash with the key name matching the controller's name.

However, even though the controller is plural, the model name has to be singular.

I fixed my problem changing users list to user:

{
    "user": [{
        "type": "test", 
        "name": "test", 
        "account_id": "2DAC41seuRwGIxTn4PQPZc"
    }, {
        "type": "test2", 
        "name": "test2", 
        "account_id": "2DAC41seuRwGIxTn4PQPZc"
    }]
}

Parameters: {"user"=>[{"type"=>"test", "name"=>"test", "account_id"=>"2DAC41seuRwGIxTn4PQPZc"}, {"type"=>"test2", "name"=>"test2", "account_id"=>"2DAC41seuRwGIxTn4PQPZc", }], "account_id"=>"2DAC41seuRwGIxTn4PQPZc"}