Mongoid Hash Field Types – Watch Out

We have blogged about Mongoid before. It’s fast, flexible, and supports Hash and Array field types. However useful those types can be, they harbor a lurking monster of deceit.

Modifying a Hash or Array that lives in one of these enumerable data types yields some strange behavior. Saves and updates can be eaten by the hungry beast of a “successful” save-and-reload on the model.

class Foo
  include Mongoid::Document
  field :stats, :type => Hash
end
  

f = Foo.create(:stats => {"bar" => {}})

f.stats["bar"]["baz"] = "qux"
# => <Foo _id: 4da45df77654aa7729000004, stats: {"bar"=>{"baz"=>"qux"}}>
f.save!
# => true

# looks good
f.stats["bar"]
# => {"baz"=>"qux"}

# oh noes, what happened?
f.reload.stats["bar"]
# => {}
  

The Why

Under the covers, Mongoid has a reference to the stats hash. When asked for stats, Mongoid creates a shallow copy via #dup. Since you are receiving a shallow copy, both stats point to the same "bar" hash. When you update a value in the "bar" hash, you’re actually updating it in both places. On save, Mongoid does an equality check to determine if anything has changed. Since both stats hashes point to the "bar", Mongoid thinks the object is clean and thus does not save!

A Workaround

A deep copy ensures that you’re not modifying Mongoid’s copy of the object’s state.

class Object
  def deep_copy
    Marshal.load(Marshal.dump(self))
  end
end
f = Foo.create(:stats => {"bar" => {}})
stats_copy = f.stats.deep_copy
stats_copy["bar"]["baz"] = "qux"

f.stats = stats_copy
f
# => #<Foo _id: 4da45f987654aa7729000005, stats: {"bar"=>{"baz"=>"qux"}}>
f.save!
# => true

# looks good
f.stats["bar"]
# => {"baz"=>"qux"}

# victory...
f.reload.stats["bar"]
# => {"baz"=>"qux"}
  

We decided to use #deep_copy for the few rare cases when this came up in our application. This could be done internally in a patch to Mongoid, but I doubt the patch would be accepted because of performance concerns.

Google Group discussion

Github Issue