Adding `num_min` parameter break order of orderable related object during creation only

Issue Summary

During creation only, adding min_num parameter to InlinePanel breaks the order of orderable related object when extra forms are added by clicking “Add {obj_name}” button.

Steps to Reproduce

1. Set min_num for InlinePanel for related links to any desirable number(e.g 2) in BlogPage class

from /wagtaildemo/demo/models.py lines: 423-429

BlogPage.content_panels = [
    FieldPanel('title', classname="full title"),
    FieldPanel('date'),
    StreamFieldPanel('body'),
    InlinePanel('carousel_items', label="Carousel items"),
    InlinePanel('related_links', label="Related links", min_num=2),
]

1. In admin interface create a new “Blog page”
2. Fill in 2 minimum required forms for related links (for visibility I titled links: ‘Link 1’, ‘Link 2’ etc)
3. Click ‘add related links’ button to add extra form
4. Fill in the extra form
5. Repeat steps 4 and 5 N-times
6. Save draft of the page
7. After saving you can see that the order of related links is incorrect: extra links (Link 3 - Link N) become first links in the list and Link 1 and Link 2 moved to the bottom.

It seems that adding min_num parameter breaks javascript. If min_num parameter is not set, the value attr of hidden input ‘ORDER’ for the first form in the formset is adding. However, when I add min_num parameter, the value attr of ‘ORDER’ for the first N required forms in the formset, is missing (is not adding), however for extra forms, the value of ‘ORDER’ is adding.

Technical details

Tried in 2 environments:

First

• Python 3.4.3
• Django==1.10.8
• wagtail==1.12.2
• Browser version: Chrome Version 61.0.3163.79 (Official Build) (64-bit)

Second

• Python 2.7.12
• Django==1.11.5
• wagtail==1.11.1
• Browser version: Chrome Version 61.0.3163.79 (Official Build) (64-bit)

[DIGIREN]

Hey everybody. I was also having this issue, and while a fix on the actual CMS end will still be required at some point, this little hacky jQuery function should be enough to get you by. Just run it on page load on the admin side. This looks for any orderables that are missing that value=“” attr that the CMS does not generate when a min_num is set, and adds it.

function fixOrderables(){
    //Finding all inputs that contain -ORDER in their ID.
    $("input[id*='-ORDER").each(function(){
        if($(this).val() === ""){
            var input_id = $(this).attr("id");
            //Getting the number out of the ID and adding 1 to construct our new value.
            var input_val = parseInt(input_id.substr(input_id.indexOf("-ORDER")-1, 1))+1;
            if (parseInt($(this).val()) != input_val){
                $(this).val(input_val);
            }
        }
    });
 }

[lb-]

IMHO - I like managing this on the serverside (option 1), reasoning is it is easier to test, more explicit in what we are providing to the clientside.