I ran into a problem where a

 / belongs_to

relation only works one way. It turned out that when querying the child from the parent, the wrong foreign_key is used. This is what my relations look like:

class Ontology::Job

  include Mongoid::Document

  has_many :ontology_synonyms, :class_name => 'Ontology::Synonym'

end

class Ontology::Synonym

  include Mongoid::Document

  belongs_to :ontology_job, :class_name => 'Ontology::Job'

end

I expect the foreign key on the child to be

 since this is what derives from the relation name, but it queries 

job_id

 instead.

irb
[36] pry(main)> oj = Ontology::Job.first
MOPED: 127.0.0.1:27017 QUERY database=ontology_development collection=ontology_jobs selector={"$query"=>{}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil (0.9291ms)
[37] pry(main)> oj.ontology_synonyms
MOPED: 127.0.0.1:27017 QUERY database=ontology_development collection=ontology_synonyms selector=

flags=[] limit=0 skip=0 batch_size=nil fields=nil (53.1111ms)
=> []

The relations metadata contains different foreign_keys depending on the direction queried. I expected them to both be 

ontology_job_id

.

irb
[23] pry(main)> Ontology::Synonym.reflect_on_association(:ontology_job).foreign_key
=> "ontology_job_id"
[24] pry(main)> Ontology::Job.reflect_on_association(:ontology_synonyms).foreign_key
=> "job_id"
`