# model_a.rb
module Space
  class ModelA
    include Mongoid::Document

    belongs_to :model_b
  end
end

# model_b.rb
module Space
  class ModelB
    include Mongoid::Document

    has_one :model_a
  end
end

# app.rb
require_relative 'model_a.rb'
require_relative 'model_b.rb'

a = Space::ModelA.create
b = Space::ModelB.create(model_a: a)

####### Result #######
Traceback (most recent call last):
	29: from app.rb:16:in `<main>'
	28: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/persistable/creatable.rb:146:in `create'
	27: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/threaded/lifecycle.rb:161:in `_creating'
	26: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/persistable/creatable.rb:150:in `block in create'
	25: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/persistable/creatable.rb:150:in `new'
	24: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/document.rb:114:in `initialize'
	23: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/threaded/lifecycle.rb:89:in `_building'
	22: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/document.rb:119:in `block in initialize'
	21: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/attributes/processing.rb:29:in `process_attributes'
	20: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/attributes/processing.rb:123:in `process_pending'
	19: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/attributes/processing.rb:135:in `process_relations'
	18: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/attributes/processing.rb:135:in `each_pair'
	17: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/attributes/processing.rb:140:in `block in process_relations'
	16: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/accessors.rb:247:in `block (2 levels) in define_setter!'
	15: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/accessors.rb:140:in `without_autobuild'
	14: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/accessors.rb:256:in `block (3 levels) in define_setter!'
	13: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/accessors.rb:25:in `__build__'
	12: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/accessors.rb:42:in `create_relation'
	11: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/referenced/has_one/buildable.rb:23:in `build'
	10: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/referenced/has_one/buildable.rb:31:in `clear_associated'
	 9: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/relatable.rb:304:in `inverse'
	 8: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/relatable.rb:121:in `inverses'
	 7: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/referenced/has_one.rb:187:in `determine_inverses'
	 6: from /usr/local/lib/ruby/gems/2.5.0/gems/mongoid-7.0.0/lib/mongoid/association/relatable.rb:160:in `klass'
	 5: from /usr/local/lib/ruby/gems/2.5.0/gems/activesupport-5.1.6/lib/active_support/core_ext/string/inflections.rb:66:in `constantize'
	 4: from /usr/local/lib/ruby/gems/2.5.0/gems/activesupport-5.1.6/lib/active_support/inflector/methods.rb:267:in `constantize'
	 3: from /usr/local/lib/ruby/gems/2.5.0/gems/activesupport-5.1.6/lib/active_support/inflector/methods.rb:267:in `inject'
	 2: from /usr/local/lib/ruby/gems/2.5.0/gems/activesupport-5.1.6/lib/active_support/inflector/methods.rb:267:in `each'
	 1: from /usr/local/lib/ruby/gems/2.5.0/gems/activesupport-5.1.6/lib/active_support/inflector/methods.rb:269:in `block in constantize'
/usr/local/lib/ruby/gems/2.5.0/gems/activesupport-5.1.6/lib/active_support/inflector/methods.rb:269:in `const_get': uninitialized constant ModelA (NameError)
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