Question

I come from a MySQL background and am trying to wrap my head around MongoDB. In particular, I’m struggling to conceptualize how I should model n:n relationships the “Mongo way.”

For this example, let’s say we have two collections: users and interests. We need to be able to represent or query for several things in our data:

• User’s interests
• User’s rating of interest, e.g. “like” or “dislike”
• Users with a given interest
• Counter (which can be incremented/decremented) of each rating of the interest
• Interest name

In MySQL, I would have created a users_interests table indexed on both user IDs and interest IDs. For the counter, I would have had separate columns for each rating type, and each time a user rated/un-rated an interest, done a transaction to ensure that the counts were never false.

I’ve tried reading about some schema designs, but to no avail.

Can you help a lost soul find the way?

Answer 1

Great question. Let me first outline a bit of how the N:N relationship works then I’ll go into detail on each of your bullet points.

N:N in MySQL normally you have your pivot table associating between your user and interests (user_interests table). In mongo you do this a bit differently. You still have a users and interest collection, however instead now, you store a list of keys under interests for a user. SO something like this:

User Collection {
      "name":"Josh",
      "user":"jsmith",
      "interests":[
           {
            "_id":12345,
            "rating":"like"
           },
           {..}..
      ]
}

By storing your interests in a list which is keyed off on your interest table, you can perform each of the actions you require. If you wanted to do a query you’d od it based on the ID which is in the interest table then do a query using the $in modifier.

Now for your interests collection I’d do the following:

User Interest {
      "_id":objectId
      "label":"Swimming",
      "count":intValue
}

When adding an interest to a users document, the count variable would then depend on the definition of your ratings. If you’re storing your ratings in a separate area (or in logic), then the value you assigned to them would be what you relate then to the int value in interest. IE: User rates it meh (which has a value of 1) then you would add 1 to the count value.

Hopefully this is helpful and has at the very least brought about some other ideas on how to structure it!

Best of luck, remember MONGO IS AWESOME.