Validate uniqueness of RelationTransition#to_status

app/models/relation_transition.rb

@@ -19,6 +19,8 @@ class RelationTransition < ApplicationRecord
 
   validates :name, good_small_text: true, uniqueness: true
 
+  validates :to_status, uniqueness: { scope: :from_status }
+
   validate :statuses_are_not_equal
 
 private

db/migrate/20190921161613_create_relation_transitions.rb

@@ -7,7 +7,9 @@ class CreateRelationTransitions < ActiveRecord::Migration[6.0]
     create_table :relation_transitions do |t|
       t.timestamps null: false
 
-      t.references :from_status, foreign_key: { to_table: :relation_statuses }
+      t.references :from_status,
+                   null: true,
+                   foreign_key: { to_table: :relation_statuses }
 
       t.references :to_status,
                    null: false,

db/migrate/20191001211809_unique_index_statuses_on_relation_transitions.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+class UniqueIndexStatusesOnRelationTransitions < ActiveRecord::Migration[6.0]
+  def change
+    reversible do |dir|
+      dir.down do
+        execute <<~SQL
+          DROP INDEX
+            index_relation_transitions_on_to_status_id_when_from_status_id_is_null
+        SQL
+      end
+
+      dir.up do
+        execute <<~SQL
+          CREATE UNIQUE INDEX
+            index_relation_transitions_on_to_status_id_when_from_status_id_is_null
+            ON relation_transitions
+            USING btree
+            (to_status_id)
+            WHERE from_status_id IS NULL;
+        SQL
+      end
+    end
+  end
+end

db/structure.sql

@@ -1433,6 +1433,13 @@ CREATE UNIQUE INDEX index_relation_transitions_on_name ON public.relation_transi
 CREATE INDEX index_relation_transitions_on_to_status_id ON public.relation_transitions USING btree (to_status_id);
 
 
+--
+-- Name: index_relation_transitions_on_to_status_id_when_from_status_id_; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE UNIQUE INDEX index_relation_transitions_on_to_status_id_when_from_status_id_ ON public.relation_transitions USING btree (to_status_id) WHERE (from_status_id IS NULL);
+
+
 --
 -- Name: index_relationships_on_from_date; Type: INDEX; Schema: public; Owner: -
 --
@@ -1717,6 +1724,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20190930205337'),
 ('20190930210852'),
 ('20190930215223'),
-('20191001022049');
+('20191001022049'),
+('20191001211809');
 
 

spec/models/relation_transition_spec.rb

@@ -28,6 +28,13 @@ RSpec.describe RelationTransition do
       is_expected.to validate_presence_of(:to_status).with_message(:required)
     end
 
+    # Does not work with relations, but I don't want to use IDs.
+    # Will wait while matcher will be upgraded.
+    xit do
+      is_expected.to \
+        validate_uniqueness_of(:to_status_id).scoped_to(:from_status_id)
+    end
+
     it { is_expected.not_to allow_value subject.from_status }
   end