Mongoid can't handle $pushes into a deeply embedded array. Consider the following code

class A
include Mongoid::Document
embeds_many :bs
end

class B
include Mongoid::Document
embedded_in :a, :inverse_of => :bs
embeds_many :cs
end

class C
include Mongoid::Document
embedded_in :b, :inverse_of => :cs
end

c = C.create(:b => B.create(:a => A.create))

which mongoid translates into this query:

db.as.update(

{"_id": "4c7ae50fc61aeb7190000028", "bs._id": "4c7ae50fc61aeb7190000029"}

, {"$push": {"bs.cs":

{"_id": "Whatever"}

}})

which fails with "can't append to array using string field name [cs]"

Instead, mongoid should use the $ positional operator (http://www.mongodb.org/display/DOCS/Updating#Updating-The%24positionaloperator) like so:

db.as.update(

{"_id": "4c7ae50fc61aeb7190000028", "bs._id": "4c7ae50fc61aeb7190000029"}

, {"$push": {"bs.$.cs":

{"_id": "Whatever"}

}})