Revise Rails 5 belongs_to associations required by default

More info: rails/rails#18233

app/models/court.rb

@@ -9,14 +9,14 @@ class Court < ActiveRecord::Base
   has_many :hearings
   has_many :decrees
 
-  belongs_to :jurisdiction, class_name: 'Court::Jurisdiction'
+  belongs_to :jurisdiction, class_name: 'Court::Jurisdiction', optional: true
   belongs_to :municipality
 
   has_many :offices, class_name: 'Court::Office'
 
-  belongs_to :information_center,       class_name: 'Court::Office'
-  belongs_to :registry_center,          class_name: 'Court::Office'
-  belongs_to :business_registry_center, class_name: 'Court::Office'
+  belongs_to :information_center,       class_name: 'Court::Office', optional: true
+  belongs_to :registry_center,          class_name: 'Court::Office', optional: true
+  belongs_to :business_registry_center, class_name: 'Court::Office', optional: true
 
   has_many :expenses, class_name: 'Court::Expense'
 

app/models/decree.rb

@@ -1,8 +1,9 @@
 class Decree < ActiveRecord::Base
   belongs_to :source
 
-  belongs_to :proceeding
-  belongs_to :court
+  # TODO consider this required in future
+  belongs_to :proceeding, optional: true
+  belongs_to :court, optional: true
 
   has_many :judgements
   has_many :judges, through: :judgements
@@ -12,8 +13,8 @@ class Decree < ActiveRecord::Base
   has_many :naturalizations, class_name: 'Decree::Naturalization'
   has_many :natures, class_name: 'Decree::Nature', through: :naturalizations
 
-  belongs_to :legislation_area
-  belongs_to :legislation_subarea
+  belongs_to :legislation_area, optional: true
+  belongs_to :legislation_subarea, optional: true
 
   has_many :legislation_usages
   has_many :legislations, through: :legislation_usages

app/models/employment.rb

@@ -1,5 +1,5 @@
 class Employment < ActiveRecord::Base
   belongs_to :court
   belongs_to :judge
-  belongs_to :judge_position, class_name: 'Judge::Position'
+  belongs_to :judge_position, class_name: 'Judge::Position', optional: true
 end

app/models/hearing.rb

@@ -1,16 +1,16 @@
 class Hearing < ActiveRecord::Base
   belongs_to :source
 
-  belongs_to :proceeding
-  belongs_to :court
+  belongs_to :proceeding, optional: true
+  belongs_to :court, optional: true
 
   has_many :judgings
   has_many :judges, through: :judgings
 
   belongs_to :type,    class_name: 'Hearing::Type',    foreign_key: :hearing_type_id
-  belongs_to :section, class_name: 'Hearing::Section', foreign_key: :hearing_section_id
-  belongs_to :subject, class_name: 'Hearing::Subject', foreign_key: :hearing_subject_id
-  belongs_to :form,    class_name: 'Hearing::Form',    foreign_key: :hearing_form_id
+  belongs_to :section, class_name: 'Hearing::Section', foreign_key: :hearing_section_id, optional: true
+  belongs_to :subject, class_name: 'Hearing::Subject', foreign_key: :hearing_subject_id, optional: true
+  belongs_to :form,    class_name: 'Hearing::Form',    foreign_key: :hearing_form_id, optional: true
 
   has_many :proposers
   has_many :opponents

app/models/judge/designation.rb

@@ -2,5 +2,5 @@ class Judge::Designation < ActiveRecord::Base
   belongs_to :source
 
   belongs_to :judge
-  belongs_to :type, class_name: 'Judge::Designation::Type', foreign_key: :judge_designation_type_id
+  belongs_to :type, class_name: 'Judge::Designation::Type', foreign_key: :judge_designation_type_id, optional: true
 end

app/models/selection_procedure.rb

@@ -1,7 +1,7 @@
 class SelectionProcedure < ActiveRecord::Base
   belongs_to :source
 
-  belongs_to :court
+  belongs_to :court, optional: true
 
   has_many :candidates, class_name: 'SelectionProcedure::Candidate'
   has_many :commissioners, class_name: 'SelectionProcedure::Commissioner'

app/models/selection_procedure/candidate.rb

@@ -1,4 +1,4 @@
 class SelectionProcedure::Candidate < ActiveRecord::Base
   belongs_to :procedure, class_name: 'SelectionProcedure', foreign_key: :selection_procedure_id
-  belongs_to :judge
+  belongs_to :judge, optional: true
 end

app/models/selection_procedure/commissioner.rb

@@ -1,4 +1,4 @@
 class SelectionProcedure::Commissioner < ActiveRecord::Base
   belongs_to :procedure, class_name: 'SelectionProcedure', foreign_key: :selection_procedure_id
-  belongs_to :judge
+  belongs_to :judge, optional: true
 end