How would you implement these queries efficiently in MongoDB?

Question

Links have one or more tags, so at first it might seem natural to embed the tags:

link = { title: 'How would you implement these queries efficiently in MongoDB?'
         url: 'http://stackoverflow.com/questions/3720972'
         tags: ['ruby', 'mongodb', 'database-schema', 'database-design', 'nosql']}

How would these queries be implemented efficiently?

• Get links that contain one or more given tags (for searching links with given tags)
• Get a list of all tags without repetition (for search box auto-completion)
• Get the most popular tags (to display top 10 tags or a tag cloud)

The idea to represent the link as above is based on the MongoNY presentation, slide 38.

Answer 1

Get links that contain "value" tag:

db.col.find({tags: "value"});

Get links that contain "val1", "val2" tags:

db.col.find({tags: { $all : [ "val1", "val2" ] }});

Get list of all tags without repetition:

db.col.distinct("tags");

Get the most popular tags - this isn't something that can be queried on an existing db, what you need to do is add a popularity field update it whenever a query fetches the document, and then do a query with the sort field set to the popularity.

Update: proposed solution for popularity feature. Try adding the following collection, let's call it tags.

doc = { tag: String, pop: Integer }

now once you do a query you collect all the tags that were shown (these can be aggregated and done asynchronously) so let's say you end up with the following tags: "tag1", "tag2", "tag3".

You then call the update method and increment the pop field value:

db.tags.update({tag: { $in: ["tag1", "tag2", "tag3"] }}, { $inc: { pop: 1 }});