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Editorial Team
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Asked: 2026-06-06T08:15:59+00:00

I’m working on a Rails app that implements some social network features as relationships,

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I’m working on a Rails app that implements some social network features as relationships, following, etc. So far everything was fine until I came across with a problem on many to many relations. As you know mongo lacks of joins, so the recommended workaround is to store the relation as an array of ids on both related documents. OK, it’s a bit redundant but it should work, let’s say:

field :followers, type: Array, default: []
field :following, type: Array, default: []


def follow!(who)
  self.followers << who.id
  who.following << self.id
  self.save
  who.save
end

That works pretty well, but this is one of those cases where we would need a transaction, uh, but mongo doesn’t support transactions. What if the id is added to the ‘followed’ followers list but not to the ‘follower’ following list? I mean, if the first document is modified properly but the second for some reason can’t be updated.

Maybe I’m too pessimistic, but there isn’t a better solution?

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1 Answer

  1. Editorial Team

    I would recommend storing relationships only in one direction, storing the users someone follows in their user document as “following”. Then if you need to query for all followers of user U1, you can query for {users.following : "U1"} Since you can have a multi-key index on an array, this query will be fast if you index this field.

    The other reason to go in that direction only is a single user has a practical limit to how many different users they may be following. But the number of followers that a really popular user may have could be close to the total number of users in your system. You want to avoid creating an array in a document that could be that large.

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