I'm doing a relationship with many to many with mongoDb and mongoose.js, i know that there is many options, my situation is this:

I've two documents, user and projects, one user can have many projects and one project can have many user, so in my case i've 4 options:

1 - An array of id_user inside project document.

2 - An array of id_project inside user document.

3 - An array of id_user inside project document && An array of id_project inside user document.

4 - A third table mapping user and project relationship(like a relational database).

The option 1 and 2 are unavailable, because, imagine in the scenario of the option 1 if i wanted to find all projects from the user, i will have to look for this user id inside every project document array of the users(traverse this array in every project), this definitely isn't a good approach.

The option 3 is good but i will have to make some kind of transaction to ensure that both documents will be written, it's not that bad, because both documents will be much more read than written

The option 4 is simpler because when i add one user to a project, it's just to add a new document with both id's(it's good solution i think, because i will don't need to care about transaction, it's a good solution?)

So, what's the best solution?

Solution 1

To the contrary, solution 1 and 2 are your best bet. Solution 3 can be considered when the update/creation frequency is very less compared to read frequency of projects and users as even though to update/create, it requires two queries, the ease of reading will be make up for that.

To choose among solution 1 and 2, you need to consider the read frequencies. Will you need projects of a user or uses of a project more frequently and choose according to that. If you feel both are of relatively the same frequency, it is better to keep the user object as less clustered as possible. Whatever option you choose, consider keeping an index on the array storing the _ids (of projects or users).

For ex.

userSchema = new Schema(
               project_ids: [{type: Schema.Types.ObjectId, ref: 'Project'}})


projectSchema = new Schema(
               user_ids: [{type: Schema.Types.ObjectId, ref: 'User'}})

Keeping an index on the array of _id will vastly improve your queries' speed on the side where you fear there will be significant overhead.

But keep the index only if this relation is an important relation with a lot of queries going on. If this is just a side feature of your project, you can do without an index too.

If the user can do lots of stuff and has lots of relations, you will be needing that user object constantly throughout your app, so if your app isn't project specific, it would be better to not put the project ids in the user schema. But as we are just putting the ids, it isn't much of an overhead anyway. No need to worry about that.

Reg index on both the arrays: Yes you can ofcourse. But when you go for solution 3, you don't need an index at all as you won't be doing a query to get the list of projects of a user or the list of users in a project. Solution 3 makes reading very easy but writing a bit cumbersome. But as you mentioned that your use case involves reading>>writing, go with solution 3 but there's always a danger of data inconsistency which you need to take care of.

Indexing just makes things faster. Go through the docs and do a bit of googling. Nothing fancy. Querying over indexed arrays is efficient than normal arrays. For ex. Let us assume you use solution 2. Store the project ids in the project_ids field.

You can get the projects of a user easily. This is straight forward.

But to get users of project1. You need to a query like this.

     //here docs will be the list of the users of project1
//The above query might be slow if the user base is large. 
//But it can be improved vastly by indexing the project_ids field in the User schema.

Similary for solution 1. Each project has user_ids field.Let us assume we have a user1. To get the projects of user we do the folowing query

      //here docs will be the projects of user1
      //But it can be improved vastly by indexing the user_ids field in the Project schema.

If you are pondering over solution 1 vs solution 2, solution 1 is better I guess. There might be cases where you need user without his projects but the chances of requiring the project without users is pretty low. But it depends on your exact use case.

Solution 2

Well solution 1 and 2 are not looking so bad! If you index array of ObjectIds, you can then directly access to what you want.

Solution 3 is looking good too, but 4, not really, you need more queries, except when you have lots of changes between relations of projects and users, but not them itself.

I prefer solution 1 with index on array. I guess lots of times you need to get project object depend on user or directly by id, so .find() will do all you want. I think it's not bad to keep user schema with minimum informations, they looks like isolate objects and you may need them for other purposes.