Flat nested set to nested structs in Go

Question

Maybe its just late in the day and this is making my brain melt.

I'm trying to convert a flat list of nested sets into a multi-dimensional nested array.

I have a bunch of structured CMS entries as follows:

entries := []TreeEntry{
        {
            Slug:  "about-us",
            Title: "About us",
            Left:  2,
            Right: 11,
            Level: 1,
        },
        {
            Slug:  "more-about-us",
            Title: "More about us",
            Left:  3,
            Right: 6,
            Level: 2,
        },
        {
            Slug:  "even-more-about-us",
            Title: "Even more about us",
            Left:  4,
            Right: 5,
            Level: 3,
        },
        {
            Slug:  "contact-us",
            Title: "Contact us",
            Left:  2,
            Right: 7,
            Level: 1,
        },
    }

And I want to unflatten them as follows:

entries := []TreeEntry{
        {
            Slug:  "about-us",
            Title: "About us",
            Left:  2,
            Right: 11,
            Level: 1,
            Children: []TreeEntry{
                {
                    Slug:  "more-about-us",
                    Title: "More about us",
                    Left:  3,
                    Right: 6,
                    Level: 2,
                    Children: []TreeEntry{
                        {
                            Slug:  "even-more-about-us",
                            Title: "Even more about us",
                            Left:  4,
                            Right: 5,
                            Level: 3,
                        },
                    },
                },
            },
        },
        {
            Slug:  "contact-us",
            Title: "Contact us",
            Left:  2,
            Right: 7,
            Level: 1,
        },
    }

The aim here is ultimately to return a menu structure with the slugs contact'ed together as appropriate, but some reason, I just cannot get my head around achieving this in Go.

Can anybody point me in the correct direction?

Edit: Added non-working example of what I've attempted:

https://play.golang.org/p/oKWo21lu__7

The results never add below the first level.

Thanks, J

Please show what you tried. Also, the desired structure based on the field values isn't clear, what are Left, Right, and Level and how are they used to build the tree? — Marc
– Marc, Commented Jan 31, 2018 at 18:03
I've tried re-implementing this, in go: stackoverflow.com/questions/16999530/… Left, Right and Level come from a database (not mine) and are the normal nested set values. — JimBlizz
– JimBlizz, Commented Jan 31, 2018 at 18:07
The link that you've provided already contains a recursive solution, you just need to translate that in go. And like Marc mention, it's easier to help if you showed us what you tried. — ahmy
– ahmy, Commented Jan 31, 2018 at 19:56
I've tried to tidy up my example, and given an idea of what i'm trying here: play.golang.org/p/oKWo21lu__7 The problem here is it never gets past the first nested level, and i'm unsure why. The data is sorted in level order from the database. — JimBlizz
– JimBlizz, Commented Feb 1, 2018 at 9:04

JimBlizz · Accepted Answer · 2018-02-01 10:03:55Z

As it turns out, I needed to use pointers to hold the child entries. The following setup works:

type TreeEntry struct {
    Id       int64        `db:"elementId" json:"id"`
    Slug     string       `db:"slug" json:"slug"`
    Title    string       `db:"title" json:"title"`
    Left     int          `db:"lft" json:"-"`
    Right    int          `db:"rgt" json:"-"`
    Level    int          `db:"level" json:"-"`
    Children []*TreeEntry `json:"children"`
}

func (t *TreeEntry) AddNestedChild(newEntry TreeEntry) {
    // If this child is one level below the current node, just add it here for now
    if newEntry.Level == t.Level+1 {
        t.Children = append(t.Children, &newEntry)
    } else {
        // Loop through the children and see if it fits anywhere
        for _, child := range t.Children {
            if newEntry.Left > child.Left && newEntry.Right < child.Right {
                child.AddNestedChild(newEntry)
                break
            }
        }
    }
}

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