Cannot use ActiveRecord helper to create associated model from parent model record

#### Environment details

- Programming language: Ruby
- OS: Mac OS
- Language runtime version: 3.1.2
- Package version: 1.2.2

#### Steps to reproduce

1. Set up a rails app with a model that has_many of another model
2. Attempt to create a record of the child model from a record of the parent model with the following syntax: `parent_model.child_models.create()`

Whenever I do this I get this error: `google-cloud-ruby/google-cloud-spanner/lib/google/cloud/spanner/convert.rb:120:in field_for_object': ArgumentError: Cannot determine type for nil values. (ActiveRecord::StatementInvalid)`