Flat Category Enabled - Bug

[tzyganu]

When I set the value of Use Flat Catalog Category to Yes from Stores->Configuration->Catalog->Frontend I don't have anymore the subcategories in the main menu in the frontend.
I only get the top level categories.
Max recursion level is the same.
Digging down I found that if I change in Magento\Catalog\Model\Observe::_addCategoriesToMenu() this

       if ($this->categoryFlatConfig->isFlatEnabled()) {
            $subcategories = (array)$category->getChildrenNodes();
        } else {
            $subcategories = $category->getChildren();
        }

to this:

 $subcategories = $category->getChildren();

everything works.
I found that with flat catalog on and off $category is always an instance of Magento\Framework\Data\Tree\Node so the if statement that checks if the flat catalog is enabled might not be needed.
I didn't add this as a pull request because I'm not sure if I'm right or not.
Please review it.
Thanks in advance.

[verklov]

Thanks, @tzyganu! The team will look at the code changes you proposed. We will get back to you once there is feedback available from the team.

[tzyganu]

@verklov I did some digging and I think the problem is much bigger.
The code I mentioned might not be the problem.
It seams that enabling the flat category flag has no effect.
I truncated all the catalog_category_flat_store_ tables, cache is off and cleared, but I still see the top level categories in the frontend.
I think that the code that loads the category collection does not take into consideration the flat flag. It always loads the categories from the EAV tables.
I assume that if this is changed, the code I mentioned in the original post should not be changed anymore.

[verklov]

@tzyganu, the fix for this issue has just been deployed to the repository. Thank you for this bug report! We are closing this issue now. Looking forward to new bug reports and suggestions from you!