Can't bind list to command object without indexing

Question

I'm trying to make the default data biding to work with a command object that has a domain object list. This are domain classes and command objects from an example project I created not my final domains.

package testbinding
import grails.validation.Validateable

@Validateable
class SelectionCommand implements Serializable {
    List<Book> books
    Author author
}

With Book and Author:

package testbinding

class Book {
    Long id
    String name

    static constraints = {
    }
}

package testbinding

class Author {

    Long id
    String name

    static constraints = {
    }
}

Controller:

def index(SelectionCommand command) {
    println command
    if (command?.hasErrors()) {
        println command?.errors
    }

    [command: command]
}

If I have a form using indexes for the book domain the binding is correct. For example:

<label>Books</label>
<input name="book[0].id" value="1"/>
<input name="book[1].id" value="2"/>
<label>Author</label>
<g:select name="author.id" value="${1L}" from="${Author.list()}" optionKey="id" optionValue="name"/>
<button type="submit">Submit</button>

This binds correctly but I need book to be a drop-down so I can't have it indexed.

When using:

<label>Books</label>
<g:select name="books" from="${Book.list()}" multiple="true" optionKey="id" optionValue="name" value="${[1L, 2L]}"/>
<label>Author</label>
<g:select name="author.id" value="${1L}" from="${Author.list()}" optionKey="id" optionValue="name"/>
<button type="submit">Submit</button>

I'm not able to get the binding correctly. I've tried with name="books" and name="books.id" and with both I get validation errors.

My sample project is using Grails 2.3.9 but I had the same issues in 2.3.11.

There is an old issue for this but this was supposed to be solved in 2.3.x.

Answer 1

Answering my own question. This apparently was still an issue in Grails 2 up to 2.4.4 (https://github.com/grails/grails-core/issues/1380).