# 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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