Question

I have got two classes

class Claim
  include Mongoid::Document
  embeds_many :claim_fields
  belongs_to :user
  ...
end

class ClaimField
  include Mongoid::Document
  embedded_in :claim
  field :title
  field :value
  ...
end

I need to fetch all unique values for claim_fields with current title for my claim through db (not by Ruby – it is too slow for thousands of records)

I’ve already tryied this

user = User.find(...)
Claim.collection.distinct("claim_fields.value", {:user_id => user.id, "claim_fields.title" => some_title})
# that is the same as user.claims.find(...).distinct("claim_fields.value")

But it returns ALL claim_fields values, and I need it to return only values for claim_fields with title that I need.

PS looks that I need some MapReduce here

Answer 1

The fundamental problem here is that MongoDB queries only return entire documents. You are filtering on claim_fields.title, but the system is returning all Claim documents that match.

You’re doing a distinct, but MongoDB treats sub-objects and Documents differently. As a result, the distinct is probably not doing what you want it to.

There are two possible solutions here:

1. Pre-calculate via M/R. (as you suggest)
2. Break these out into two collections.

Regarding #2, there is no requirement for embedding objects as you have. Embedded should be done based on the queries you plan to perform most. So if this is a common query, then it’s fair to make these separate documents.