How to model many-to-many relationships in MongoDB (for a MySQL user)

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

Answer 1

To maintain a global count of the ratings of each interest, you will need a separate and independent collection where you update (add or subtract) ratings using atomic update operators as and when like/dislike actions for interests are performed by users.

You can store each User's interest as an array of sub-documents within the User collection itself.

The JSON structure of this data would be something similar to:

db.User
{
    name: 'joe',
    ....,
    interests : [{ name: 'swimming', rating: 10},
              { name: 'cooking', rating: 22 }
              ]
}

Now you can query on the internal keys using:

> db.User.find( { "interests.name" : "cooking" } )

This will return users who have a particular interest.

Answer 2

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.