Question

difficult title for a simple issue 🙂

say we have a list of books
they are in different categories, these categories are an array property on the book

we want to transform this json, into a list of unique categories, with the books under the category

first off: the json:

[
    {
        "description":"book 1 description",
        "title":"book 1 title",
        "id":"4",
        "logo":"",
        "image":"",
        "categories":[
            {
                "id":"1",
                "title":"Logistiek"
            },{
                "id":"2",
                "title":"Finances"
            }
        ]
    },
    {
        "description":"book 2 description",
        "title":"book 2 title",
        "id":"1",
        "logo":"",
        "image":"",
        "categories":[
            {
                "id":"3",
                "title":"Telecom"
            }
        ]
    },
    {
        "description":"book 3 description",
        "title":"book 3 title",
        "id":"2",
        "logo":"",
        "image":"",
        "categories":[
            {
                "id":"3",
                "title":"Telecom"
            }
        ]
    },
    {
        "description":"book 4 description",
        "title":"book 4 title",
        "id":"3",
        "logo":"",
        "image":"",
        "categories":[
            {
                "id":"2",
                "title":"Finances"
            }
        ]
    }
]

now what i managed myself:

i started by mapping off all the categories:

var data = {} // lets say all json is inhere...
var res = _(data).map(function(m){
    return m.categories;
});

this I flatten into 1 array of categories (because now its an array per book.

res = _(res).flatten();

this gives me an array of all category items, though this has doubles in it.
now i’m not getting much further than this yet.

i tried using the union method before flattening but that didnt help out
i tried the uniq on the bigger array but i think i have to break em down into separate arrays for the uniq to work

i’m kind of stuck getting the unique values out of that array of categories.
after that I can manage to add the books under the categories

If anyone got some ideas, or maybe tell me that i’m doing this completely wrong 🙂 go ahead tell me, if i can do it shorter or with better performance by going another direction feel free to throw it at me.

update1

ok, now i got a little further but i’m pretty sure it’s not ideal, (too many steps, i get a feeling getting a unique list could go quicker than these steps)

// get all categorie arrays
var res = _(data).map(function(m){
    return m.categories;
});

// flatten them
res = _(res).flatten();

// reduce to unique ID array
var catIds = _(res).pluck('id');
catIds = _(catIds).uniq();

// from here on create an array with unique categories
var cats = [];

_(catIds).each(function(cId){
    var s = _(res).filter(function(c){
        return c.id === cId;
    });
    cats.push(_(s).first());
});

can I do this quicker?

see jsFiddle in action…
http://jsfiddle.net/saelfaer/JVxGm/

update 2

ok, i got further, thanks to the help from you guys below,
but i still feel like using 2 eaches is not the best way to get to the end.

var json = [] // lets say the above json is in this variable.
var books = _(json).map(function(book) {
    var cats = book.categories;
    delete book.categories;
    return _(cats).map(function(cat) {
        return _({}).extend(book, { category: cat });
    });
});
books = _(books).flatten();

var booksPerCategory = [];
_(books).each(function(book){
    if(!_(booksPerCategory).any(function(cat){
        return cat.id === book.category.id;
    }))
    { booksPerCategory.push(book.category); }
});

_(booksPerCategory).each(function(cat){
    var mods = _(books).filter(function(book){
        return book.category.id === cat.id;
    });
    cat['modules'] = mods;
});

you can see what i wanted to recieve: the booksByCategory array
and what i got via the help from below: the books object
both in this jsfiddle: http://jsfiddle.net/saelfaer/yWCgt/

Answer 1

You could do something like this:

var books = ​_(json).map(function(book) {
    var cats = book.categories;
    delete book.categories;
    return _(cats).map(function(cat) {
        return _({}).extend(book, { category: cat.id });
    });
});
books = _(books).flatten();
books = _(books).groupBy(function(book) { return book.category });

Demo: http://jsfiddle.net/ambiguous/jaL8n/

If you only want a slice of each book’s attributes then you can adjust this:

return _({}).extend(book, { category: cat.id });

accordingly; for example, if you just want the titles and category IDs:

return _(cats).map(function(cat) {
    return {
        category: cat.id,
        title:    book.title
    };
});

Demo: http://jsfiddle.net/ambiguous/EFLDR/

You don’t need to explicitly generate a set of unique category IDs, you just have to arrange your data appropriately and everything falls out on its own (as is common with functional approaches).

---

UPDATE: Based on the comments, I think you just need to add a reduce after the flatten instead of a groupBy:

var books = _(json).map(function(book) {
    var cats = book.categories;
    delete book.categories;
    return _(cats).map(function(category) {
        return {
            category: category,
            book:     book
        };
    });
});
books = _(books).flatten();
books = _(books).reduce(function(h, b) {
    if(!h[b.category.id])
        h[b.category.id] = _(b.category).extend({ modules: [ ] });
    h[b.category.id].modules.push(b.book);
    return h;
}, { });

Demo: http://jsfiddle.net/ambiguous/vJqTz/

You could also use chain and some named functions to make it easier to read:

function rearrange(book) {
    var cats = book.categories;
    delete book.categories;
    return _(cats).map(function(category) {
        return {
            category: category,
            book:     book
        };
    });
}

function collect_into(h, b) {
    if(!h[b.category.id])
        h[b.category.id] = _(b.category).extend({ modules: [ ] });
    h[b.category.id].modules.push(b.book);
    return h;
}

var books = _.chain(json).
    map(rearrange).
    flatten().
    reduce(collect_into, { }).
    value();

Demo: http://jsfiddle.net/ambiguous/kSU7v/