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Mongoid Pitfall

Vinh  mongoid ruby

Default value and callback

I recently started to work with Rails and Mongoid. I don’t have much experience with ActiveRecord and this is the first job that I actually use Rails.

The other day we have a query that looks like this:

array.each do |user_id|
  User.find(user_id)
end

The array contains 20 elements. And A find by id should be very fast but above query run in ~10s.

At first I tried to turn it into something like this

User.where(:id.in => array).each 

But it is still superslow. Turning out MongoiD will auto load all relations and this user has lots of association so Mongoid will fetch all kind of relations. This array contains a list of heavy users in our app with lots of related data.

The fix is simply:

User.without(:relation).find(user_id)

The lession is that we should always benchmark code instead of blindly guessing which parts of code is slow. If I benchmark them right at the beginning I would have known exactly the problem much quicker.

The second thing is default value. Value is evaluated instantly at run time, when the file is loaded if the value wasn’t a closure.

Let’s say you have this:

field :foo, type: Time, default: Time.now.utc

Now, the value of everydocument will be exactly same value at the time the file is loaded and evaluated by RubyVM.

The right way to do this is using a callback so that the value is evaluate lazily at runtime

The other day I see no data in an admin ui which supposed to always have data. I fired up Rails console, and run query:

Check.where(type: 'http').count

It returns 0:

But I do see some document with that:

Check.first.type

I open the mongo shell to check real data and the field was missing. Turning out, the default masked the problem, when data is missing, default is kicked in and field it in.