Rails multi-level relation destroyed when updating - ruby-on-rails

Question

I have a four-level deep model structure: Domain > Subject > Device > Property
class Domain < ApplicationRecord
has_many :subjects
end
class Subject < ApplicationRecord
has_many :devices
belongs_to: :domain
end
class Device < ApplicationRecord
has_many :properties
belongs_to: :subject
end
class Property < ApplicationRecord
belongs_to :device
end
controller code
def update
result = #subject.update(parameters)
if result
render json: #subject
else
render_errors(#subject.errors)
end
end
#subject is retrieved as a before action, by querying the model tree from domain onwards, using domain_id and id parameters for domain and subject respectively. parameters is simply a hash of parameters, e.g. {name: :new_name}
When updating a Subject, the relation to domain is lost, i.e. domain_id is set to NUL by rails. The entire model tree below subject will also be disconnected from the parent domain as a result.
When removing has_many: :devices from the Subject model, everything works as expected. I just want to update a subject and preserve the relation to the parent domain. How would I achieve this with the model described above?
EDIT 1 - Added log of both situations.
Log with full relational model (that results in the bug)
Domain Load (0.5ms) SELECT "domains".* FROM "domains" WHERE "domains"."name" = ? LIMIT ? [["name", "Manatree"], ["LIMIT", 1]]
CACHE (0.0ms) SELECT "domains".* FROM "domains" WHERE "domains"."name" = ? LIMIT ? [["name", "Manatree"], ["LIMIT", 1]]
Subject Load (0.0ms) SELECT "subjects".* FROM "subjects" WHERE "subjects"."domain_id" = ? AND "subjects"."name" = ? LIMIT ? [["domain_id", 3], ["name", "s1"], ["LIMIT", 1]]
Subject Load (0.5ms) SELECT "subjects".* FROM "subjects" WHERE "subjects"."id" = ? LIMIT ? [["id", 5], ["LIMIT", 1]]
(0.0ms) begin transaction
Domain Load (0.0ms) SELECT "domains".* FROM "domains" WHERE "domains"."id" = ? LIMIT ? [["id", 3], ["LIMIT", 1]]
SQL (7.1ms) UPDATE "subjects" SET "domain_id" = NULL WHERE "subjects"."domain_id" = ? AND "subjects"."id" = 5 [["domain_id", 3]]
Subject Exists (0.0ms) SELECT 1 AS one FROM "subjects" WHERE "subjects"."domain_id" = ? AND "subjects"."name" = ? LIMIT ? [["domain_id", 3], ["name", "s2"], ["LIMIT", 1]]
SQL (1.0ms) UPDATE "subjects" SET "name" = ?, "updated_at" = ? WHERE "subjects"."id" = ? [["name", "s2"], ["updated_at", "2017-07-31 08:46:38.171240"], ["id", 5]]
(7.5ms) commit transaction
Subject Load (0.0ms) SELECT "subjects".* FROM "subjects"
Completed 200 OK in 30ms (Views: 0.5ms | ActiveRecord: 16.6ms)
Log when removing belongs_to: :devices from Subject model
Domain Load (0.0ms) SELECT "domains".* FROM "domains" WHERE "domains"."name" = ? LIMIT ? [["name", "Manatree"], ["LIMIT", 1]]
CACHE (0.0ms) SELECT "domains".* FROM "domains" WHERE "domains"."name" = ? LIMIT ? [["name", "Manatree"], ["LIMIT", 1]]
Subject Load (0.5ms) SELECT "subjects".* FROM "subjects" WHERE "subjects"."domain_id" = ? AND "subjects"."name" = ? LIMIT ? [["domain_id", 3], ["name", "s3"], ["LIMIT", 1]]
Subject Load (0.5ms) SELECT "subjects".* FROM "subjects" WHERE "subjects"."id" = ? LIMIT ? [["id", 6], ["LIMIT", 1]]
(0.0ms) begin transaction
Domain Load (0.5ms) SELECT "domains".* FROM "domains" WHERE "domains"."id" = ? LIMIT ? [["id", 3], ["LIMIT", 1]]
SQL (6.5ms) UPDATE "subjects" SET "name" = ?, "updated_at" = ? WHERE "subjects"."id" = ? [["name", "s4"], ["updated_at", "2017-07-31 08:48:57.962218"], ["id", 6]]
(7.0ms) commit transaction
Subject Load (0.0ms) SELECT "subjects".* FROM "subjects"
Completed 200 OK in 25ms (Views: 0.4ms | ActiveRecord: 15.1ms)
Edit 2 - There might be something wrong with the seed data...
domainA = Domain.create(name: :company)
s1 = domainA.subjects.create(name: :subject)
# domainA.save
d1 = s1.devices.create(name: :device)
# s1.save
p1 = d1.properties.create(name: :prop1, property_type: :double, value: 10.0)
p2 = d1.properties.create(name: :prop2, property_type: :string, value: :on)
p3 = d1.properties.create(name: :prop3, property_type: :string, value: :Lamp)
# d1.save
domainA.save

Answer 1

When removing has_many: :devices from the Subject model, everything
works as expected.
Going on with your models, Your Device model is flawed. You have belongs_to :device inside Device. Perhaps you should have belongs_to :subject as per the associations. This could have lead to your current problem. Try changing the Device model like so
class Device < ApplicationRecord
has_many :properties
belongs_to: :subject
end