Question

This is an interesting pattern I found using mongoid (3.0.0), which I suspect is a bug.

1.9.3p194 :007 > products = Product.order_by([:_id, :asc ]).limit(5)
 => #<Mongoid::Criteria
   selector: {},
   options:  {:sort=>{"_id"=>1}, :limit=>5},
   class:    Product,
   embedded: false>

1.9.3p194 :008 > products.map(&:_id)
 => ["500fa5614f6d3a23d0000002", "500fa5614f6d3a23d0000003", "500fa5614f6d3a23d0000004", "500fa5614f6d3a23d0000005", "500fa5614f6d3a23d0000006"] 

So far so good! However, if I issue the following – I get weird results.

1.9.3p194 :012 > products.count
 => 3654017 

This shows me all the product count instead of 5 (since i have :limit => 5)

1.9.3p194 :012 > Product.count
 => 3654017 

Now the even more weird part:

1.9.3p194 :010 > products.last
 => #<Product _id: 504952620a5e2323460000aa, _type: nil, ... >

This should have been _id: 500fa5614f6d3a23d0000006. Now, if I try to map the ids again, I get:

1.9.3p194 :019 > products.map(&:id)
 => ["504952620a5e2323460000aa", "504952620a5e2323460000a9", "504952620a5e2323460000a8", "5049524f0a5e2323460000a7", "504950ab0a5e2323460000a6"] 

This has changed the criteria entirely! However, I get proper results with this:

1.9.3p194 :008 > products = Product.order_by([:_id, :asc ]).limit(5)
 => #<Mongoid::Criteria
  selector: {},
  options:  {:sort=>{"_id"=>1}, :limit=>5},
  class:    Product,
  embedded: false>

1.9.3p194 :028 > products[0].id
 => "500fa5614f6d3a23d0000002" 
1.9.3p194 :029 > products[-1].id
 => "500fa5614f6d3a23d0000006" 

This does seem to be related to Mongoid 3.0.0 though. Any ideas?

Answer 1

First keep in Mind that Mongoid has a lazy loading functionnality: the query will be trigerred at the last moment possible.

Let’s dig your issues:

• last: it sets limit to -1 so it will override your previous setting. To get your expected behavior, you must oblige Mongoid to make the query using to_a: products = Product.order_by([:_id, :asc ]).limit(5).to_a.last

• count: if you want to respect the limit, use count(true)