Mongoid::Contextual#each delegated to context.each, but context is nil - ruby

Question

I have a Sinatra application connecting to MongoDB via Mongoid. It was working fine until a point where I started getting a weird error that I can't find anywhere on stackoverflow or anywhere.
Note that the same code is deployed on Heroku and it works fine with the same database version Mongo 3.0.7, ruby version 2.0.0 which I tried locally as well as 2.2.3. I get the error when I run shotgun through the web and when running Rspec:
Bible::Book#get_book_id is case-insensitive
Failure/Error: book[0]._id
Module::DelegationError:
Mongoid::Contextual#each delegated to context.each, but context is nil: #<Mongoid::Criteria
selector: {"title"=>"genesis"}
options: {}
class: Bible::Book
embedded: false>
# /Users/issa/.rvm/gems/ruby-2.2.3/gems/mongoid-5.0.0/lib/mongoid/contextual.rb:20:in `rescue in each'
# /Users/issa/.rvm/gems/ruby-2.2.3/gems/mongoid-5.0.0/lib/mongoid/contextual.rb:20:in `each'
# /Users/issa/.rvm/gems/ruby-2.2.3/gems/mongoid-5.0.0/lib/mongoid/criteria.rb:554:in `entries'
# /Users/issa/.rvm/gems/ruby-2.2.3/gems/mongoid-5.0.0/lib/mongoid/criteria.rb:554:in `method_missing'
# ./app/bible/book.rb:52:in `get_book_id'
# ./spec/bible/book_spec.rb:11:in `block (3 levels) in <top (required)>'
# ------------------
# --- Caused by: ---
# NoMethodError:
# undefined method `each' for nil:NilClass
# /Users/issa/.rvm/gems/ruby-2.2.3/gems/mongo-2.1.2/lib/mongo/cluster.rb:114:in `initialize'
My Rspec code:
RSpec.describe Bible::Book do
describe "#get_book_id" do
it "finds the id of genesis" do
expect(Bible::Book.get_book_id 'genesis').to eq 1
end
end
end
The code implementation is this:
module Bible
class Book
include Mongoid::Document
store_in collection: "bible_books"
field :title, type: String
field :testament, type: String
...
def self.get_book_id book_title
book = where(:title => book_title.downcase.strip)
return unless book
book[0]._id
end
end
end
My Gemfile has this part for MongoDB:
# MongoDB
...
gem 'mongoid', '~> 5.0'
gem 'bson_ext'
gem 'bson'
It's really strange that I get this error only on my machine, and just started out of the blue. I am using OS X El Capitan. I can access the mongodb collection from command line without issues and can run the same equivalent query to get the result I want:
> use bible
switched to db bible
> db.bible_books.find({"title": "genesis"})
{ "_id" : 1, "title" : "genesis", "testament" : "old", "chapters" : 50, "doc_type" : "book", "canon_type" : "canonical", "canon_order" : 1, "full_name" : { "en" : "The book of Genesis", "ar" : "سفر التكوين" }, "short_name" : { "en" : "Genesis", "ar" : "التكوين" }, "abbr_name" : { "ar" : "تك", "en" : "Gen" } }
I tried other collections, other queries, other variations, and still get the same error. I reverted to an older code from github that worked for sure, still the same issue, so it's clear to me that there's nothing wrong in my code or configuration. I tried different ruby versions from 2.0.0 - 2.2.3, different mongoid versions from 4 - 5 ... again the same versions of code and installation work fine on Heroku, but not my machine :(

Answer 1

I got similar error when wrong production configuration url was specified for mongoid: uri: <%= ENV['MONGOLAB_URI'] %>

Answer 2

I'm not sure if this is the source of your problem but I see a couple odd things in your get_book_id class method.
First of all, this will never leave book.nil?:
book = where(:title => book_title.downcase.strip)
That will leave you with a Mongoid::Contextual in book, the query may or may not find anything but book won't be nil in either case. That means that your:
return unless book
on the next line doesn't do anything useful. That also means that book[0] can be nil in here:
book[0]._id
You're also calling the _id method when you'd usually call id.
I think your method would be better off like this:
def self.get_book_id book_title
book = find_by(:title => book_title.downcase.strip)
return unless book
book.id
end
find_by(query) is just a short way of saying where(query).first so book = find_by(...) will leave you with a nil in book if it can't find anything and a single Bible::Book in book if it can find something.
If you have ActiveSupport available, you could also say:
find_by(:title => book_title.downcase.strip).try(:id)
to hide the nil check. You could also throw in an only call to only pull the _id out of MongoDB:
book = only(:id).find_by(:title => book_title.downcase.strip)
return unless book
book.id
# or, if ActiveSupport is around, just this:
only(:id).find_by(:title => book_title.downcase.strip).try(:id)