Silverstripe 3 - GridFieldExtensions MultiClass Adding - silverstripe

Question

I'm trying to use https://github.com/silverstripe-australia/silverstripe-gridfieldextensions/ to create a gridfield where I can add different types of dataobjects.
Sadly I can't figure out how to write the correct code for that on my class where I want the gridfield.
Could someone please point me in the right direction?
UPDATE:
Based on your answers, I now have the following structure
class ModularPage extends Page {
private static $has_many = array(
'Sections' => 'MP_Section',
'Galleries' => 'MP_Gallery',
'Paragraphs' => 'MP_Paragraph'
);
public function getCMSFields() {
...
$fields->addFieldToTab('Root.Main', $mutli_grid = GridField::create('Sections', 'Sektionen', $this->Sections(), MultiClassGrid::create(15)));
...
}
}
class MP_Section extends DataObject {
private static $has_one = array(
'Section' => 'MP_Section',
'ModularPage' => 'ModularPage'
);
}
class MP_Gallery extends MP_Section {
private static $has_one = array(
'Section' => 'MP_Section',
'ModularPage' => 'ModularPage'
);
}
So far, so good? Is this right until now?
Cause If I want to add for example a gallery, I receive the following error
[User Error] Couldn't run query: SELECT DISTINCT "MP_Section"."ID", "MP_Section"."SortID" FROM "MP_Section" LEFT JOIN "MP_Gallery" ON "MP_Gallery"."ID" = "MP_Section"."ID" LEFT JOIN "MP_Paragraph" ON "MP_Paragraph"."ID" = "MP_Section"."ID" WHERE ("ModularPageID" = '13') ORDER BY "MP_Section"."SortID" ASC LIMIT 9223372036854775807 Column 'ModularPageID' in where clause is ambiguous

Answer 1

Here is how I usually setup my GridField:
$c = GridFieldConfig_RelationEditor::create();
$c->removeComponentsByType('GridFieldAddNewButton')
->addComponent(new GridFieldAddNewMultiClass())
;
$c->getComponentByType('GridFieldAddNewMultiClass')
->setClasses(array(
'SectionThemesBlock' => SectionThemesBlock::get_section_type(),
'SectionFeaturedCourse' => SectionFeaturedCourse::get_section_type(),
'SectionCallForAction' => SectionCallForAction::get_section_type(),
'SectionContactSheet' => SectionContactSheet::get_section_type()
//....
));
$f = GridField::create('Sections', "Sections", $this->Sections(), $c);
$fields->addFieldToTab("Root.Sections", $f);
Based on the GridFieldConfig_RelationEditor, just remove GridFieldAddNewButton then add GridFieldAddNewMultiClass. Then configure the component to know which classes to have available in the dropdown to create. All those SectionThemesBlock, SectionFeaturedCourse etc extend a common Section dataObject as base. The get_section_type() function is a custom static function on the Section dataobject to get a nice looking name in the dropdown and not have to type it manually all the time....
The basics of the Section dataobject looks like so:
class Section extends DataObject {
public static function get_section_type()
{
return trim(preg_replace('/([A-Z])/', ' $1', str_ireplace('Section', '', get_called_class())));
}
//...
}
And the page where that gridField goes and that has the relation defined on:
class Page extends SiteTree {
//...
private static $has_many = array(
'Slides' => 'Slide'
);
//...
}

Answer 2

Something like this should work
$config = new GridFieldConfig_RecordEditor();
$config->addComponent(new GridFieldAddNewMultiClass());
...
$grid = GridField::create('Grid', 'Grid', $this->GalleryItems(), $config);
You need three DataObjects:
GalleryItem extends DataObject{}
FooGalleryItem extends GalleryItem{}
BarGalleryItem extends GalleryItem{}