Use dedicated sequence record instead of deal with issues with ids and uuids when using the previous approach

test/models/account/external_id_sequence_test.rb

@@ -0,0 +1,42 @@
+require "test_helper"
+
+class Account::ExternalIdSequenceTest < ActiveSupport::TestCase
+  setup do
+    Account::ExternalIdSequence.delete_all
+  end
+
+  test ".next returns sequential values" do
+    first_value = Account::ExternalIdSequence.next
+    second_value = Account::ExternalIdSequence.next
+    third_value = Account::ExternalIdSequence.next
+
+    assert_equal first_value + 1, second_value
+    assert_equal second_value + 1, third_value
+  end
+
+  test ".next initializes from maximum external_account_id" do
+    max_id = Account.maximum(:external_account_id) || 0
+
+    first_value = Account::ExternalIdSequence.next
+
+    assert_equal max_id + 1, first_value
+  end
+
+  test ".next creates single sequence record" do
+    3.times { Account::ExternalIdSequence.next }
+
+    assert_equal 1, Account::ExternalIdSequence.count
+  end
+
+  test ".next is concurrency-safe" do
+    values = 20.times.map do
+      Thread.new do
+        Account::ExternalIdSequence.next
+      end
+    end.map(&:value)
+
+    assert_equal 20, values.uniq.size, "All values should be unique"
+    assert_equal values.min..values.max, values.sort.first..values.sort.last
+    assert_equal 20, values.max - values.min + 1, "Values should be sequential with no gaps"
+  end
+end

test/models/account_test.rb

@@ -69,12 +69,13 @@ class AccountTest < ActiveSupport::TestCase
     assert_equal account1.external_account_id + 1, account2.external_account_id
   end
 
-  test "external_account_id can be overridden without creating sequence entry" do
-    custom_id = Account.last.id + 99999
+  test "external_account_id can be overridden" do
+    custom_id = 999999
+    sequence_value_before = Account::ExternalIdSequence.first_or_create!(value: 0).value
 
-    assert_no_difference -> { ExternalAccount.count } do
-      account = Account.create!(name: "Custom ID Account", external_account_id: custom_id)
-      assert_equal custom_id, account.external_account_id
-    end
+    account = Account.create!(name: "Custom ID Account", external_account_id: custom_id)
+
+    assert_equal custom_id, account.external_account_id
+    assert_equal sequence_value_before, Account::ExternalIdSequence.value
   end
 end