I did this:

class Response
include Mongoid::Document
embedded_in :resource
field :value, type: String
end

class Resource
include Mongoid::Document
field :url, type: String

1. embeds_many :responses
   end

Note that I had the embeds_many commented out. A silly mistake, I know, but I just wanted to share the debugging process – in the hopes that maybe I can be enlightened – or hopefully, we can add a useful error message.

In my app, resource.valid? caused a mysterious error:

NoMethodError in ResponsesController#create

undefined method `name' for nil:NilClass
Rails.root: /Users/david/dev/sample-app

Application Trace | Framework Trace | Full Trace
mongoid (3.0.0.rc) lib/mongoid/validations/uniqueness.rb:245:in `validate_embedded'
mongoid (3.0.0.rc) lib/mongoid/validations/uniqueness.rb:47:in `validate_each'
activemodel (3.2.6) lib/active_model/validator.rb:153:in `block in validate'
activemodel (3.2.6) lib/active_model/validator.rb:150:in `each'
activemodel (3.2.6) lib/active_model/validator.rb:150:in `validate'
activesupport (3.2.6) lib/active_support/callbacks.rb:310:in `_callback_before_17'
activesupport (3.2.6) lib/active_support/callbacks.rb:418:in `_run*3393481399181526745*validate*1919942954378861761*callbacks'
activesupport (3.2.6) lib/active_support/callbacks.rb:405:in `__run_callback'
activesupport (3.2.6) lib/active_support/callbacks.rb:385:in `_run_validate_callbacks'
activesupport (3.2.6) lib/active_support/callbacks.rb:81:in `run_callbacks'

I took a look at line 245.

def validate_embedded(document, attribute, value)
return if skip_validation?(document)
relation = document._parent.send(document.metadata.name)
criteria = create_criteria(relation, document, attribute, value)
add_error(document, attribute, value) if criteria.count > 1
end

So it turns out metadata was nil. But from there, I am left wondering what the problem might be.

So, to make a long story short (too late) – I know that I just needed to uncomment the line above – is there an easy way on Mongoid's end to make a clear error message for something like this?