In a recent project I had a $many_many relation between TeamMember and TeamGroup where each member can have a different role per group. I solved this with a manymany relation and extraFields, but how can I tell my GridField to show me a nice form field for editing the "TeamRole" extra field?

My code (with namespaces stripped) is like:

class TeamMember extends DataObject
{
[..]

    private static $many_many = [
        'TeamGroups' => TeamGroupPage::class,
    ];

    private static $many_many_extraFields = [
        'TeamGroups' => [
            'TeamRole' => 'Varchar',
            'SortOrder' => 'Int'
        ]
    ];
}

and the counterpart

class TeamGroupPage extends Page
{
[..]

    private static $belongs_many_many = [
        'TeamMembers' => TeamGroup::class,
    ];


}

When I'm on the TeamGroupPage and try to add or edit a TeamMember we need to modify the GridFieldDetailForm to to add the extra fields:

//on class TeamGroupPage

public function getCMSFields()
    {
        $fields = parent::getCMSFields();
        $conf = GridFieldConfig_RelationEditor::create(20);

        $teamGrid = Gridfield::create(
            'TeamMembers',
            'Team Members',
            $this->TeamMembers(),
            $conf
        );

        $detailFormComponent = $teamGrid->getConfig()->getComponentByType(GridFieldDetailForm::class);
        $detailFormComponent->setItemEditFormCallback(function ($form, $itemrequest) {
            $record = $itemrequest->record;
            $roleField = TextField::create('ManyMany[TeamRole]', 'Team Role', $record->TeamRole);
            $form->Fields()->add($roleField);
        });

        $fields->addFieldToTab('Root.TeamMembers', $teamGrid);

        return $fields;
    }

We get the GridFieldDetailForm component of the grid field's config and modify it by adding a field to TeamMember's CMSFields. With the method setItemEditFormCallback() we can add an anonymous function to add the needed field(s). Manymany extraFields are saved using the ManyMany[FieldName] syntax, so we need ManyMany[TeamRole] as the FieldName; we also need to grab the current value from $itemrequest->record.

Now John Doe can be "Boss" of the A-Team and normal member of team B...