UX: GridFieldConfig_RelationEditor 'steals' relation on has_many without warning

[jonom]

Affected Version

4.6.0

Description

I noticed today that a CMS user probably can't tell the difference between a has_many and many_many Gridfield (using GridFieldConfig_RelationEditor) and thus likely wouldn't realise that the 'Link existing' function behaves differently depending on the type of relationship. I was caught off guard today thinking I was saving myself time by reusing content in a many_many relationship, but then realised it was a has_many when I saw that most of the 'holders' I was working on were empty.

A possible solution could be that if you select an item which is already linked to something else in a has_many relationship you could get a modal confirmation message like:

"That {Thing} is already linked to {Title of object}. Linking it to this {Thing} will unlink it from {Title of object}."

Cancel Duplicate instead Proceed

Related: WPP-Public/akqa-nz-silverstripe-menumanager#41

Steps to Reproduce

Use GridFieldConfig_RelationEditor on a has_many relation.

[jonom]

This came up on a project today and I implemented a little hack to just remove the 'link existing' field from GridFieldConfig_RelationEditor on has many lists, since I don't think it should be there. I customised GridFieldAddExistingAutocompleter::getHTMLFragments() with:

if ($gridField->list instanceof HasManyList) {
    return "";
}