function oneToOne(...args) {
    return new OneToOne(...args);
}
Defines a one-to-one relationship. In database terms, this is a foreign key with the
added restriction that only one entity can point to single target entity.

The arguments are the same as with `fk`. If `relatedName` is not supplied,
the source model name in lowercase will be used. Note that with the one-to-one
relationship, the `relatedName` should be in singular, not plural.


@param {string|Class<Model>|Object} options - The target Model class, its `modelName`
                                              attribute or an options object that
                                              contains either as the `to` key.
@param {string|Class<Model>} options.to - The target Model class or its `modelName` attribute.
@param {string} [options.as] - Name for the new accessor defined for this field. If you don't
                               supply this, the key that this field is defined under will be
                               overridden.
@param {string} [options.relatedName] - The property name that will be used to access the source
                                        model instance referencing the target model instance.
@param {string} [relatedName] - The property name that will be used to access the source
                                model instance referencing the target model instance
@return {OneToOne}