How to implement post tags in Mongo?

Question

I\'m toying with Mongo to make a SO-like pet project, and I want to implement post tags. Each tag has a name and a slug (string to be used as an id in the URL), and a post h

Answer 1

If the tags you use and their respective slugs are unlikely to change, I think your second approach is the better one. However I would suggest a small change - rather than storing an array of [name, slug], make the fields explicit by creating a tag subdocument as in this example post document:

{
    "_id" : ObjectId("4ee33229d8854784468cda7e"),
    "title" : "My Post",
    "content" : "This is a post with some tags",
    "tags" : [
        {
            "name" : "meta",
            "slug" : "34589734"
        },
        {
            "name" : "post",
            "slug" : "34asd97x"
        },
    ]
}

You can then query for posts with a particular tag using dot notation like this:

db.test.find({ "tags.name" : "meta"})

Because tags is an array, mongo is clever enough to match the query against any element of the array rather than the array as a whole, and dot-notation allows you to match against a particular field.

To query for posts not containing a specific tag, use $ne:

db.test.find({ "tags.name" : { $ne : "fish" }})

And to query for posts containing one tag but not the other, use $and:

db.test.find({ $and : [{ "tags.name" : { $ne : "fish"}}, {"tags.name" : "meta"}]})

Hope this helps!