Schema for User Ratings - Key/Value DB

Question

We\'re using MongoDB and I\'m figuring out a schema for storing Ratings.

• Ratings will have values of 1-5.
• I want to store other values such as from

Answer 1

I would do it a bit different: Have a User class and a Rating class and aggregate the number of ratings and rating average.

The Rating class

This is a bit of pseudo code, but the meaning should be obvious.

{
  _id:ObjectId(…),
  rating: Integer,
  rater: User._id
  rated: User._id
  date: ISODate()
}

In order to do the aggregation efficiently, you should at least create an index over rated:

db.ratings.ensureIndex({rated:1})

Now, you can decide between to approaches: either, you calculate the number of ratings and the average let's say once an hour and store it in an collection, let's say rate_averages, or you calculate those values on demand.

Precalculated

db.ratings.aggregate(
  // Aggregation
  [{
     $order: {
      _id: "$rated",
      ratings: { $sum:1 },
      average: { $avg: "$rating" }
    },
    {$out:'rate_averages'}
  ]
)

A document in the rate_averages collection will then look like this:

{
  _id:User._id,
  ratings: Integer,
  average: Float
}

and is easily queryable for the individual user's values, as _id is indexed automatically.

On demand

You'd use the same rating and almost the same aggregation query, except that we add a $match stage so we only work with the values for the user we want to know the stats for and leave out the $out stage and have the document to be returned directly:

db.ratings.aggregate([
  {
    $match:{ rated: <_id of the user we want the values for> },
  },
  {
    $order: {
      _id: "$rated",
      ratings: { $sum:1 },
      average: { $avg: "$rating" }
  }
])

which would return a single document as shown for the user in question.

With this approach and a proper data model, you can even do such things as "How many ratings were given by a specific user on a given date?" or "What are the most active raters/the most rated?" quite easily.

Please read the aggregation framework docs for further details. You might find the data modeling docs useful, too.